Here are the one’s I use:
CustomizeGoogle: https://addons.mozilla.org/en-US/firefox/addon/743/
Better Gmail: https://addons.mozilla.org/en-US/firefox/addon/6076/
Webmail Notifier: https://addons.mozilla.org/en-US/firefox/addon/4490/

Also useful for free online storage: GSpace: https://addons.mozilla.org/en-US/firefox/addon/1593/

@premiersoupir Not really Google related per say. But one I can’t live without is Ubiquity