feat(extension): add Firefox WebExtension support

- Add a neutral WebExtension API adapter that uses Firefox browser.* or Chromium chrome.* without mutating globals.
- Switch extension runtime code to the adapter and add Firefox-specific typings for tabs, windows, tab groups, storage, scripting, and native messaging ports.
- Fix Firefox temporary add-on instructions to load the packaged manifest with background.scripts instead of the Chromium service worker manifest.
- Detect Firefox in clients.list via runtime.getBrowserInfo and keep Chromium user-agent fallback support.
- Make navigate.open wait briefly for Firefox to replace initial about:blank with the requested URL.
- Add JS coverage for API selection, clients.list browser detection, and Firefox navigate.open URL polling.
- Bump package and extension version to 0.15.2.

tests/test_cli.py

@@ -140,6 +140,10 @@ def test_install_writes_firefox_allowed_extensions(tmp_path):
     }
   ]
   assert "about:debugging#/runtime/this-firefox" in result.output
+  assert "npm run package:extension:firefox" in result.output
+  assert "dist/extension-package-firefo" in result.output
+  assert "x/manifest.json" in result.output
+  assert "Do not select extension/manifest.json" in result.output
   assert "Firefox extension ID" in result.output
 
 def test_install_windows_registers_native_host(tmp_path):

tests/test_extension_error_page_handling.py

@@ -74,7 +74,7 @@ def test_large_extension_operations_yield_between_batches():
     assert "GENTLE_OPERATION_PAUSE_MS" in core
     assert "itemCount >= 300" in core
     assert "itemCount >= 100" in core
-    assert "chrome.tabs.query({ audible: true })" in core
+    assert "api.tabs.query({ audible: true })" in core
     # The centralized batch loop drives cancellation + progress + throttled yield.
     assert "processInBatches" in core
     assert "throwIfJobCancelled(progress.job)" in core
@@ -93,7 +93,7 @@ def test_large_extension_operations_yield_between_batches():
     assert "yieldForLargeOperation(createdTabs.length" in session
     assert "getLargeOperationThrottle" in session
     assert "runLargeOperation(\"session.load\"" in session
-    assert "chrome.tabs.discard" in session
+    assert "api.tabs.discard" in session
     assert "lazyPlaceholderUrl" in session
     assert "activateLazyTab" in session
     assert "lazySessionTabs" in session
@@ -136,8 +136,8 @@ def test_built_extension_avoids_static_firefox_unsupported_tab_group_api_refs():
     assert "chrome.tabGroups" not in background
     assert "chrome.tabs.group" not in background
     assert "chrome.tabs.ungroup" not in background
-    assert 'chrome["tabGroups"' in background
-    assert 'chrome.tabs["group"' in background
+    assert 'webExtApi["tabGroups"' in background
+    assert 'webExtApi.tabs["group"' in background
 
 def test_built_extension_avoids_direct_eval_token_for_firefox_linter():
     background = read_built_background()
@@ -164,9 +164,9 @@ def test_session_autosave_is_debounced_and_non_overlapping():
     assert "autoSaveSignature" in autosave
     # AutoSaveManager binds the handlers as instance fields (this.*), so the
     # add/removeListener references stay identity-stable across enable/disable.
-    assert "chrome.tabs.onUpdated.addListener(this.autoSaveUpdatedHandler)" in autosave
-    assert "chrome.tabs.onCreated.addListener(this.autoSaveHandler)" in autosave
-    assert "chrome.tabs.onMoved.addListener(this.autoSaveHandler)" in autosave
+    assert "api.tabs.onUpdated.addListener(this.autoSaveUpdatedHandler)" in autosave
+    assert "api.tabs.onCreated.addListener(this.autoSaveHandler)" in autosave
+    assert "api.tabs.onMoved.addListener(this.autoSaveHandler)" in autosave
     assert "if (!(\"url\" in changeInfo)) return;" in autosave
     assert "setTimeout(() => this.runAutoSave(), delayMs)" in autosave
     assert "clearTimeout(this.autoSaveTimer)" in autosave