Feature Request: Add support for Firefox Add-on #2

Since Firefox fully supports Manifest V3 now, making it compatible should be pretty straightforward.
We'd mainly just need to:

1. Add browser_specific_settings (with a Gecko ID) in manifest.json.
2. Include Mozilla's webextension-polyfill so we can safely use Promise-based browser.* APIs.

I will submit a PR to make it happen.