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.

browser_cli/commands/install.py

@@ -78,9 +78,14 @@ def cmd_install(browser):
   console.print("\n[bold]Step 1:[/bold] Load the extension in your browser")
   console.print(f"  1. Open [cyan]{ext_url}[/cyan]")
   if browser == "firefox":
-    console.print("  2. Click [bold]Load Temporary Add-on...[/bold]")
-    console.print(f"  3. Select: [cyan]{Path(__file__).parent.parent.parent / 'extension' / 'manifest.json'}[/cyan]")
-    console.print(f"  4. Firefox extension ID is [cyan]{FIREFOX_EXTENSION_ID}[/cyan]")
+    repo_root = Path(__file__).parent.parent.parent
+    firefox_manifest = repo_root / "dist" / "extension-package-firefox" / "manifest.json"
+    console.print("  2. Build the Firefox-compatible temporary extension:")
+    console.print("     [cyan]npm run package:extension:firefox[/cyan]")
+    console.print("  3. Click [bold]Load Temporary Add-on...[/bold]")
+    console.print(f"  4. Select: [cyan]{firefox_manifest}[/cyan]")
+    console.print("     Do not select extension/manifest.json; Firefox currently rejects background.service_worker there.")
+    console.print(f"  5. Firefox extension ID is [cyan]{FIREFOX_EXTENSION_ID}[/cyan]")
     console.print("     Note: Firefox support is experimental; tab-group commands require browser tab group APIs.\n")
   else:
     console.print("  2. Enable [bold]Developer mode[/bold] (top-right toggle)")