diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 902d374..2e79737 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -377,12 +377,14 @@ jobs:
           TAG="${{ github.event.inputs.release_tag }}"
           [ -z "$TAG" ] && TAG="${GITHUB_REF_NAME}"
           echo "tag=$TAG" >> "$GITHUB_OUTPUT"
+          # bare revision number for the release title: firefox-9 -> 9
+          echo "num=${TAG#firefox-}" >> "$GITHUB_OUTPUT"
           echo "publishing DRAFT release for tag: $TAG"
       - name: Create DRAFT release with all assets
         uses: softprops/action-gh-release@3bb12739c298aeb8a4eeaf626c5b8d85266b0e65 # v2
         with:
           tag_name: ${{ steps.tag.outputs.tag }}
-          name: invisible_firefox (150.0.1) rev ${{ steps.tag.outputs.tag }}
+          name: invisible_firefox (150.0.1) rev ${{ steps.tag.outputs.num }}
           draft: true
           prerelease: false
           fail_on_unmatched_files: true
diff --git a/src/invisible_playwright/_geo.py b/src/invisible_playwright/_geo.py
index 02971e1..7423b2b 100644
--- a/src/invisible_playwright/_geo.py
+++ b/src/invisible_playwright/_geo.py
@@ -22,7 +22,7 @@ On failure:
 from __future__ import annotations
 
 import ipaddress
-from typing import Any, Dict, Optional
+from typing import Any, Dict, NamedTuple, Optional
 from urllib.parse import quote
 
 import requests
@@ -136,22 +136,76 @@ def ip_to_timezone(ip: str, mmdb_path: Any) -> str:
     return tz
 
 
+class SessionGeo(NamedTuple):
+    """Geo facts resolved once per session from a single egress round-trip.
+
+    ``timezone`` follows the precedence in the module docstring.
+    ``egress_ip`` is the proxy egress IP (the IP the *outside world* sees) when
+    a proxy is set, else ``None`` — it feeds the WebRTC srflx override, which is
+    only meaningful behind a proxy (a direct connection's real STUN already
+    reports the truthful public IP, so we leave it alone).
+    """
+
+    timezone: str
+    egress_ip: Optional[str]
+
+
+def prepare_session_geo(
+    timezone: str, proxy: Optional[Dict[str, str]]
+) -> SessionGeo:
+    """Resolve the session timezone AND the proxy egress IP in ONE round-trip.
+
+    The egress IP is discovered once and reused for both the timezone mapping
+    (when ``timezone`` is ``""``/``"auto"``) and the WebRTC public-IP override.
+    Timezone precedence is identical to :func:`resolve_session_timezone`; the
+    egress IP is best-effort for the WebRTC side (a discovery failure that the
+    timezone path doesn't need won't break the launch — but if the timezone
+    path *does* need it behind a proxy, that path still fails loudly).
+    """
+    from .download import ensure_geoip_mmdb
+
+    tz = (timezone or "").strip()
+    proxy_set = _proxy_is_set(proxy)
+
+    # One discovery, reused below. Behind a proxy we always want the egress IP
+    # (for WebRTC) regardless of the timezone setting.
+    egress_ip: Optional[str] = None
+    egress_err: Optional[Exception] = None
+    if proxy_set:
+        try:
+            egress_ip = discover_egress_ip(proxy)
+        except Exception as exc:  # noqa: BLE001
+            egress_err = exc
+
+    # Timezone resolution — same precedence as resolve_session_timezone.
+    if tz and tz.lower() != "auto":
+        return SessionGeo(tz, egress_ip)  # explicit IANA wins
+    try:
+        ip = egress_ip if proxy_set else discover_egress_ip(None)
+        if ip is None:  # proxy set but discovery failed above
+            raise egress_err or GeoTimezoneError("egress IP discovery failed")
+        return SessionGeo(ip_to_timezone(ip, ensure_geoip_mmdb()), egress_ip)
+    except Exception:
+        if proxy_set:
+            raise  # fail-early behind a proxy (timezone_mismatch trap)
+        return SessionGeo("", None)  # no proxy: host TZ is a safe fallback
+
+
 def resolve_session_timezone(
     timezone: str, proxy: Optional[Dict[str, str]]
 ) -> str:
     """Map the user's ``timezone`` setting to a concrete IANA zone (or ``""``).
 
-    See the module docstring for the full precedence table. ``""``/``"auto"``
-    ALWAYS resolve from the egress IP (proxy egress if a proxy is set, else the
-    host's own public IP). On failure: with a proxy we raise
-    :class:`GeoTimezoneError` (never silently use the host TZ behind a foreign
-    proxy); without a proxy we fall back to ``""`` (host TZ) so a transient
-    lookup failure can't break the launch.
+    Timezone-only path (no WebRTC side effects): an explicit IANA zone wins and
+    triggers NO network call; ``""``/``"auto"`` resolve from the egress IP. The
+    launch path uses :func:`prepare_session_geo` instead (which additionally
+    returns the egress IP for WebRTC); this standalone resolver is kept for
+    third-party integrations that only want the zone. See the module docstring
+    for the precedence table.
     """
     tz = (timezone or "").strip()
     if tz and tz.lower() != "auto":
-        return tz  # explicit IANA wins
-    # "" or "auto" → always resolve from the egress IP.
+        return tz  # explicit IANA wins — no egress lookup
     from .download import ensure_geoip_mmdb
 
     proxy_set = _proxy_is_set(proxy)
diff --git a/src/invisible_playwright/async_api.py b/src/invisible_playwright/async_api.py
index 70a7aeb..ce08608 100644
--- a/src/invisible_playwright/async_api.py
+++ b/src/invisible_playwright/async_api.py
@@ -9,7 +9,7 @@ from typing import Any, Dict, Optional, Union
 from playwright.async_api import Browser, BrowserContext, Playwright, async_playwright
 
 from ._fpforge import Profile, generate_profile
-from ._geo import resolve_session_timezone
+from ._geo import prepare_session_geo
 from ._headless import make_virtual_display
 from ._proxy import configure_proxy as _configure_proxy_shared
 from .download import ensure_binary
@@ -73,16 +73,20 @@ class InvisiblePlaywright:
         self._browser: Optional[Browser] = None
         self._persistent_context: Optional[BrowserContext] = None
         self._virtual_display: Any = None
+        # Proxy egress IP (WebRTC srflx override); discovered in __aenter__.
+        self._webrtc_egress_ip: Optional[str] = None
 
     async def __aenter__(self) -> Union[Browser, BrowserContext]:
         import sys as _sys
-        # Resolve timezone="auto" (and the proxy-set-but-unset default) to a
-        # concrete IANA zone before anything reads self._timezone. Run the
-        # blocking geo lookup off the event loop. Fail-early if a proxy is set
-        # but the egress zone can't be resolved.
-        self._timezone = await asyncio.to_thread(
-            resolve_session_timezone, self._timezone, self._proxy
+        # Resolve timezone="auto" AND discover the proxy egress IP in one
+        # round-trip, off the event loop, before anything reads self._timezone
+        # or builds prefs/env. Fail-early if a proxy is set but the egress
+        # can't be resolved.
+        _geo = await asyncio.to_thread(
+            prepare_session_geo, self._timezone, self._proxy
         )
+        self._timezone = _geo.timezone
+        self._webrtc_egress_ip = _geo.egress_ip
         executable = self._binary_path or ensure_binary()
         prefs = translate_profile_to_prefs(
             self._profile,
@@ -203,12 +207,16 @@ class InvisiblePlaywright:
         env = _os.environ.copy()
         if self._timezone:
             env["TZ"] = _tz_env(self._timezone)
-        # Propagate STEALTHFOX_WEBRTC_PUBLIC_IP if the process set it — read
-        # by nICEr's nr_stealth_bridge to inject a synthetic srflx candidate
-        # matching the proxy egress IP. This avoids the StaticPref IPC
-        # propagation timing issue between parent and socket processes.
-        if _os.environ.get("STEALTHFOX_WEBRTC_PUBLIC_IP"):
-            env["STEALTHFOX_WEBRTC_PUBLIC_IP"] = _os.environ["STEALTHFOX_WEBRTC_PUBLIC_IP"]
+        # WebRTC srflx override: feed nICEr's nr_stealth_bridge the proxy egress
+        # IP (caller's explicit env var wins, else the IP auto-discovered in
+        # __aenter__) and drop IPv6 from gathering behind a proxy.
+        webrtc_ip = (
+            _os.environ.get("STEALTHFOX_WEBRTC_PUBLIC_IP")
+            or self._webrtc_egress_ip
+        )
+        if webrtc_ip:
+            env["STEALTHFOX_WEBRTC_PUBLIC_IP"] = webrtc_ip
+            env["STEALTHFOX_WEBRTC_DISABLE_IPV6"] = "1"
         return env
 
     def _resolve_headless(self) -> bool:
diff --git a/src/invisible_playwright/launcher.py b/src/invisible_playwright/launcher.py
index 15055ee..226debf 100644
--- a/src/invisible_playwright/launcher.py
+++ b/src/invisible_playwright/launcher.py
@@ -8,7 +8,7 @@ from typing import Any, Dict, Optional, Union
 from playwright.sync_api import Browser, BrowserContext, Playwright, sync_playwright
 
 from ._fpforge import Profile, generate_profile
-from ._geo import resolve_session_timezone
+from ._geo import prepare_session_geo
 from ._headless import make_virtual_display
 from ._proxy import configure_proxy as _configure_proxy_shared
 from .download import ensure_binary
@@ -183,12 +183,19 @@ class InvisiblePlaywright:
         self._browser: Optional[Browser] = None
         self._persistent_context: Optional[BrowserContext] = None
         self._virtual_display: Any = None
+        # Proxy egress IP, discovered at launch (see __enter__). Feeds the
+        # WebRTC srflx override so the candidate matches the proxy IP, not the
+        # real host IP. None when no proxy is set.
+        self._webrtc_egress_ip: Optional[str] = None
 
     def __enter__(self) -> Union[Browser, BrowserContext]:
         # Resolve timezone="auto" (and the proxy-set-but-unset default) to a
-        # concrete IANA zone before anything reads self._timezone. Fail-early
-        # if a proxy is set but the egress zone can't be resolved.
-        self._timezone = resolve_session_timezone(self._timezone, self._proxy)
+        # concrete IANA zone AND discover the proxy egress IP — one round-trip,
+        # before anything reads self._timezone or builds prefs/env. Fail-early
+        # if a proxy is set but the egress can't be resolved.
+        _geo = prepare_session_geo(self._timezone, self._proxy)
+        self._timezone = _geo.timezone
+        self._webrtc_egress_ip = _geo.egress_ip
         executable = self._binary_path or ensure_binary()
         prefs = self._build_prefs()
         playwright_proxy = _configure_proxy_shared(self._proxy, prefs)
@@ -354,12 +361,19 @@ class InvisiblePlaywright:
         env = _os.environ.copy()
         if self._timezone:
             env["TZ"] = _tz_env(self._timezone)
-        # Propagate STEALTHFOX_WEBRTC_PUBLIC_IP if the process set it — read
-        # by nICEr's nr_stealth_bridge to inject a synthetic srflx candidate
-        # matching the proxy egress IP. This avoids the StaticPref IPC
-        # propagation timing issue between parent and socket processes.
-        if _os.environ.get("STEALTHFOX_WEBRTC_PUBLIC_IP"):
-            env["STEALTHFOX_WEBRTC_PUBLIC_IP"] = _os.environ["STEALTHFOX_WEBRTC_PUBLIC_IP"]
+        # WebRTC srflx override: feed nICEr's nr_stealth_bridge the proxy egress
+        # IP so the srflx candidate matches the proxy (not the real host the
+        # UDP STUN would otherwise leak). An explicit env var set by the caller
+        # wins; otherwise we use the egress IP auto-discovered in __enter__.
+        # Behind a proxy we also drop IPv6 from gathering (the disableIPv6 pref
+        # is dead on FF150 — the bridge filter is the real switch).
+        webrtc_ip = (
+            _os.environ.get("STEALTHFOX_WEBRTC_PUBLIC_IP")
+            or self._webrtc_egress_ip
+        )
+        if webrtc_ip:
+            env["STEALTHFOX_WEBRTC_PUBLIC_IP"] = webrtc_ip
+            env["STEALTHFOX_WEBRTC_DISABLE_IPV6"] = "1"
         return env
 
     def _resolve_headless(self) -> bool:
diff --git a/src/invisible_playwright/prefs.py b/src/invisible_playwright/prefs.py
index 4f0a15d..34ddb2f 100644
--- a/src/invisible_playwright/prefs.py
+++ b/src/invisible_playwright/prefs.py
@@ -208,15 +208,21 @@ _BASELINE: Dict[str, Any] = {
     "privacy.fingerprintingProtection.pbmode":                       False,
     "privacy.fingerprintingProtection.remoteOverrides.enabled":      False,
 
-    # WebRTC: enabled, no public IP leak.
-    # obfuscate_host_addresses=false: our C++ injection handles candidate
-    # selection; mDNS causes mDNS-IPC to hang in sandboxed content processes.
-    # disableIPv6=true keeps IPv6 out of gathering (less entropy, no IPv6 leak).
+    # WebRTC: enabled, looks like a real Firefox behind NAT, no real-IP leak.
+    # obfuscate_host_addresses=true → host candidate is `<uuid>.local` mDNS,
+    #   exactly like vanilla Firefox (BrowserLeaks "No Leak", Local IP "-").
+    #   The mDNS-IPC hang feared on older builds does NOT reproduce on FF150.
+    # The proxy-egress srflx is injected by our C++ (srflx swap §17 + fallback
+    #   §17.B), fed the egress IP via STEALTHFOX_WEBRTC_PUBLIC_IP from
+    #   launcher._build_env (auto-discovered from the proxy).
+    # IPv6: media.peerconnection.ice.disableIPv6 is DEAD on FF150 (read by no
+    #   ICE-gathering code). The real switch is our zoom.stealth.webrtc.disable_ipv6
+    #   (nICEr addrs.cpp filter) + the STEALTHFOX_WEBRTC_DISABLE_IPV6 env.
     "media.peerconnection.enabled":                       True,
     "media.peerconnection.ice.no_host":                   False,
     "media.peerconnection.ice.default_address_only":      False,
-    "media.peerconnection.ice.obfuscate_host_addresses":  False,
-    "media.peerconnection.ice.disableIPv6":               True,
+    "media.peerconnection.ice.obfuscate_host_addresses":  True,
+    "zoom.stealth.webrtc.disable_ipv6":                   True,
     "media.peerconnection.ice.proxy_only":                False,
     "media.peerconnection.ice.relay_only":                False,
     "media.peerconnection.use_document_iceservers":       True,
@@ -551,12 +557,17 @@ def translate_profile_to_prefs(
     prefs["privacy.spoof_english"]     = 0
 
     if timezone:
-        prefs["zoom.stealth.timezone"] = timezone
+        # juggler.timezone.override is the SOLE source of truth read by the C++
+        # timezone chain (BrowsingContext::Attach/DidSet, ContentChild). The old
+        # zoom.stealth.timezone pref was declared in the yaml but read by NO
+        # code — dropped here on 2026-06-10 (see 20-our-patches.md §8).
         prefs["juggler.timezone.override"] = timezone
 
-    # Cross-process seed (canvas noise + DWrite gamma share this).
+    # Cross-process seed (canvas noise + DWrite gamma share this). Only
+    # zoom.stealth.fpp.hw_seed is read by the C++; the old zoom.stealth.seed
+    # alias was never declared in the yaml and read by nothing — dropped
+    # 2026-06-10.
     prefs["zoom.stealth.fpp.hw_seed"] = profile.seed
-    prefs["zoom.stealth.seed"]        = profile.seed
 
     # Synthetic host ICE candidate — injected by C++ when addr_ct==0 (SOCKS5
     # proxy suppresses all local addresses so Firefox can't gather host cands).
diff --git a/tests/test_geo.py b/tests/test_geo.py
index 39ef5ee..f56322f 100644
--- a/tests/test_geo.py
+++ b/tests/test_geo.py
@@ -16,6 +16,7 @@ from invisible_playwright._geo import (
     _proxy_is_set,
     discover_egress_ip,
     ip_to_timezone,
+    prepare_session_geo,
     resolve_session_timezone,
 )
 
@@ -286,3 +287,39 @@ def test_resolve_proxy_failure_raises(monkeypatch):
         resolve_session_timezone("auto", SOCKS)
     with pytest.raises(GeoTimezoneError):
         resolve_session_timezone("", SOCKS)
+
+
+# ──────────────────────────────────────────────────────────────────────
+#  prepare_session_geo — one round-trip for BOTH timezone + the WebRTC
+#  egress IP. The egress feeds the srflx override (only behind a proxy).
+# ──────────────────────────────────────────────────────────────────────
+@pytest.mark.unit
+def test_prepare_geo_egress_present_behind_proxy(stub_egress):
+    geo = prepare_session_geo("auto", SOCKS)
+    assert geo.timezone == "America/New_York"
+    assert geo.egress_ip == "203.0.113.7"  # discovered for WebRTC
+    assert stub_egress["proxy_arg"] == SOCKS
+
+
+@pytest.mark.unit
+def test_prepare_geo_egress_present_even_with_explicit_tz(stub_egress):
+    # explicit IANA zone still needs the egress for WebRTC behind a proxy.
+    geo = prepare_session_geo("Asia/Tokyo", SOCKS)
+    assert geo.timezone == "Asia/Tokyo"
+    assert geo.egress_ip == "203.0.113.7"
+    assert stub_egress["called"] is True
+
+
+@pytest.mark.unit
+def test_prepare_geo_no_egress_without_proxy(stub_egress):
+    # no proxy → no WebRTC override (real STUN already tells the truth).
+    geo = prepare_session_geo("auto", None)
+    assert geo.timezone == "America/New_York"
+    assert geo.egress_ip is None
+
+
+@pytest.mark.unit
+def test_prepare_geo_timezone_matches_resolve_session_timezone(stub_egress):
+    # the thin tz wrapper must stay equivalent to prepare_session_geo().timezone
+    for tz, proxy in [("Asia/Tokyo", SOCKS), ("auto", HTTP), ("", None)]:
+        assert prepare_session_geo(tz, proxy).timezone == resolve_session_timezone(tz, proxy)
diff --git a/tests/test_integration.py b/tests/test_integration.py
index 1da7621..7abd55f 100644
--- a/tests/test_integration.py
+++ b/tests/test_integration.py
@@ -48,7 +48,6 @@ _REQUIRED_PREFS_KEYS = (
     "intl.accept_languages",
     "general.useragent.locale",
     "intl.locale.requested",
-    "zoom.stealth.seed",
     "zoom.stealth.fpp.hw_seed",
     "zoom.stealth.webrtc.host_ip",
     "zoom.stealth.webgl.renderer",
diff --git a/tests/test_launcher_helpers.py b/tests/test_launcher_helpers.py
index 5122e88..590736a 100644
--- a/tests/test_launcher_helpers.py
+++ b/tests/test_launcher_helpers.py
@@ -169,3 +169,38 @@ def test_default_context_includes_locale_when_set():
 def test_default_context_omits_locale_when_empty():
     ip = InvisiblePlaywright(seed=42, locale="")
     assert "locale" not in ip._default_context_kwargs()
+
+
+# ── InvisiblePlaywright._build_env — WebRTC egress auto-derive ─────────
+# Locks the 2026-06-10 fix: behind a proxy the launcher feeds the discovered
+# egress IP to nICEr (srflx override) + drops IPv6. Without it, a proxied
+# session's WebRTC silently fell back to leaking/blocking. Runs in tests.yml.
+
+
+@pytest.mark.unit
+def test_build_env_injects_webrtc_egress_when_discovered():
+    ip = InvisiblePlaywright(seed=42)
+    ip._webrtc_egress_ip = "203.0.113.9"  # what __enter__ resolves behind a proxy
+    env = ip._build_env()
+    assert env["STEALTHFOX_WEBRTC_PUBLIC_IP"] == "203.0.113.9"
+    assert env["STEALTHFOX_WEBRTC_DISABLE_IPV6"] == "1"
+
+
+@pytest.mark.unit
+def test_build_env_no_webrtc_keys_without_proxy(monkeypatch):
+    monkeypatch.delenv("STEALTHFOX_WEBRTC_PUBLIC_IP", raising=False)
+    ip = InvisiblePlaywright(seed=42)
+    ip._webrtc_egress_ip = None  # no proxy → real STUN already truthful
+    env = ip._build_env()
+    assert "STEALTHFOX_WEBRTC_PUBLIC_IP" not in env
+    assert "STEALTHFOX_WEBRTC_DISABLE_IPV6" not in env
+
+
+@pytest.mark.unit
+def test_build_env_caller_env_override_wins(monkeypatch):
+    monkeypatch.setenv("STEALTHFOX_WEBRTC_PUBLIC_IP", "198.51.100.5")
+    ip = InvisiblePlaywright(seed=42)
+    ip._webrtc_egress_ip = "203.0.113.9"  # auto-discovered
+    env = ip._build_env()
+    assert env["STEALTHFOX_WEBRTC_PUBLIC_IP"] == "198.51.100.5"  # caller wins
+    assert env["STEALTHFOX_WEBRTC_DISABLE_IPV6"] == "1"
diff --git a/tests/test_prefs.py b/tests/test_prefs.py
index fa27345..ae088c8 100644
--- a/tests/test_prefs.py
+++ b/tests/test_prefs.py
@@ -158,21 +158,22 @@ def test_webgl_extensions_cleared_on_windows(monkeypatch):
 
 
 @pytest.mark.unit
-def test_timezone_set_propagates_to_both_keys():
-    # TZ1
+def test_timezone_set_uses_juggler_pref():
+    # TZ1 — juggler.timezone.override is the sole C++-read timezone pref;
+    # the old zoom.stealth.timezone alias (orphan) must NOT be reintroduced.
     p = generate_profile(seed=42)
     prefs = translate_profile_to_prefs(p, timezone="America/New_York")
-    assert prefs["zoom.stealth.timezone"] == "America/New_York"
     assert prefs["juggler.timezone.override"] == "America/New_York"
+    assert "zoom.stealth.timezone" not in prefs
 
 
 @pytest.mark.unit
-def test_timezone_empty_omits_both_keys():
+def test_timezone_empty_omits_the_key():
     # TZ2
     p = generate_profile(seed=42)
     prefs = translate_profile_to_prefs(p, timezone="")
-    assert "zoom.stealth.timezone" not in prefs
     assert "juggler.timezone.override" not in prefs
+    assert "zoom.stealth.timezone" not in prefs
 
 
 # ──────────────────────────────────────────────────────────────────────
@@ -200,10 +201,10 @@ def test_extra_prefs_none_value_deletes_key():
 
 @pytest.mark.unit
 def test_extra_prefs_overrides_existing_key():
-    # EP3
+    # EP3 — override a real baseline key (hw_seed is the live cross-process seed)
     p = generate_profile(seed=42)
-    prefs = translate_profile_to_prefs(p, extra_prefs={"zoom.stealth.seed": 999})
-    assert prefs["zoom.stealth.seed"] == 999
+    prefs = translate_profile_to_prefs(p, extra_prefs={"zoom.stealth.fpp.hw_seed": 999})
+    assert prefs["zoom.stealth.fpp.hw_seed"] == 999
 
 
 @pytest.mark.unit
diff --git a/tests/test_webrtc_realness.py b/tests/test_webrtc_realness.py
index afa0736..61d487f 100644
--- a/tests/test_webrtc_realness.py
+++ b/tests/test_webrtc_realness.py
@@ -214,6 +214,43 @@ def test_mdns_host_is_invisible_to_creep_resolver():
     assert creep_get_ipaddress("v=0\r\nc=IN IP4 0.0.0.0\r\n" f"a={HOST_MDNS}\r\n") is None
 
 
+# ──────────────────────────────────────────────────────────────────────────
+#  SHIPPED-BASELINE guard — the cheap unit test that would have caught the
+#  2026-06-10 gap (baseline obfuscate=False, dead disableIPv6, orphan prefs).
+#  These lock the shipped wrapper config to the manually-validated one so a
+#  future edit / merge can't silently un-ship it. Run in tests.yml.
+# ──────────────────────────────────────────────────────────────────────────
+from invisible_playwright._fpforge import generate_profile  # noqa: E402
+from invisible_playwright.prefs import translate_profile_to_prefs  # noqa: E402
+
+
+@pytest.mark.unit
+def test_shipped_webrtc_baseline_is_the_validated_config():
+    prefs = translate_profile_to_prefs(generate_profile(seed=42))
+    # host candidate must be mDNS .local like vanilla Firefox (manually
+    # validated on BrowserLeaks/CreepJS through a residential proxy) — not a
+    # raw LAN IP.
+    assert prefs["media.peerconnection.ice.obfuscate_host_addresses"] is True
+    # IPv6 dropped via OUR live filter pref; the native pref is dead on FF150
+    # and must not be relied upon (or re-introduced as if it worked).
+    assert prefs["zoom.stealth.webrtc.disable_ipv6"] is True
+    assert "media.peerconnection.ice.disableIPv6" not in prefs
+    # peerconnection stays ON (a disabled WebRTC is itself a tell).
+    assert prefs["media.peerconnection.enabled"] is True
+
+
+@pytest.mark.unit
+def test_no_orphan_prefs_in_baseline():
+    """zoom.stealth.timezone / zoom.stealth.seed are read by NO C++ — they must
+    not be written (juggler.timezone.override + zoom.stealth.fpp.hw_seed are the
+    real ones). Guards against re-introducing a pref the binary ignores."""
+    prefs = translate_profile_to_prefs(generate_profile(seed=42), timezone="America/Chicago")
+    assert "zoom.stealth.timezone" not in prefs
+    assert "zoom.stealth.seed" not in prefs
+    assert prefs["juggler.timezone.override"] == "America/Chicago"
+    assert "zoom.stealth.fpp.hw_seed" in prefs
+
+
 # ──────────────────────────────────────────────────────────────────────────
 #  Fake-proxy infrastructure for e2e: a tiny TCP-only SOCKS5 server.
 # ──────────────────────────────────────────────────────────────────────────