Add Firefox extension test suite (WDIO v9)

- 26 tests: popup (8), options (9), navigation (3), visual (5)
- Storage-seeding strategy for API mocking (BiDi mock.respond() broken in Firefox)
- WDIO v9 + geckodriver + @wdio/visual-service
- Firefox screenshot baselines (auto-saved)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: richardkmichael

package-lock.json

@@ -17,15 +17,22 @@
     "test:chrome": "playwright test --project=chromium",
     "test:visual": "playwright test --grep @visual",
     "test:visual:update": "playwright test --grep @visual --update-snapshots",
+    "test:firefox": "wdio run wdio.conf.js",
     "package": "npm run build"
   },
   "dependencies": {
     "@github/relative-time-element": "5.0.0"
   },
   "devDependencies": {
     "@playwright/test": "^1.57.0",
+    "@wdio/cli": "^9.23.3",
+    "@wdio/local-runner": "^9.23.3",
+    "@wdio/mocha-framework": "^9.23.3",
+    "@wdio/spec-reporter": "^9.23.3",
+    "@wdio/visual-service": "^9.1.3",
     "chokidar": "^5.0.0",
     "dotenv": "^17.2.3",
+    "geckodriver": "^6.1.0",
     "web-ext": "^9.0.0"
   }
 }