feat: add Firefox extension support

- Add Firefox as an install target with native messaging manifest support.
- Generate Firefox-specific extension packages with Gecko metadata and AMO-compatible manifest transforms.
- Keep tab group commands available in Firefox through dynamic tab group API helpers.
- Avoid Firefox linter warnings for static tab group API references and direct eval tokens.
- Add Firefox packaging and installer regression coverage.
- Bump the package and extension version to 0.15.1.

tests/test_extension_error_page_handling.py

@@ -122,6 +122,22 @@ def test_tab_activation_open_and_merge_do_not_steal_audible_video_window():
     assert "skippedAudibleWindows" in tabs
     assert "const target = movableWindows.find(w => w.focused) || movableWindows[0];" in tabs
 
+def test_built_extension_avoids_static_firefox_unsupported_tab_group_api_refs():
+    background = (ROOT / "extension" / "background.js").read_text()
+
+    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
+
+def test_built_extension_avoids_direct_eval_token_for_firefox_linter():
+    background = (ROOT / "extension" / "background.js").read_text()
+
+    assert "(0, eval)(" not in background
+    assert "eval(" not in background
+    assert 'globalThis["eval"]' in background
+
 def test_session_autosave_is_debounced_and_non_overlapping():
     # The autosave lifecycle moved out of session.ts into a dedicated
     # AutoSaveManager (autosave.ts) during the structure refactor; the shared