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.

package.json

@@ -8,7 +8,8 @@
     "test:extension": "npm run build:tests && node --disable-warning=ExperimentalWarning --test extension/test-dist/*.test.mjs",
     "check:extension": "tsc -p tsconfig.json --noEmit && npm run build:extension && node --check extension/background.js && npm run test:extension",
     "package:extension": "npm run build:extension && python scripts/package_extension.py",
-    "package:extension:webstore": "npm run build:extension && python scripts/package_extension.py --webstore"
+    "package:extension:webstore": "npm run build:extension && python scripts/package_extension.py --webstore",
+    "package:extension:firefox": "npm run build:extension && python scripts/package_extension.py --firefox"
   },
   "devDependencies": {
     "@types/chrome": "^0.1.40",