Favorite interface for mail clients are Airmail (macOS), Mailbird (Windows), geary (Linux).
The thing they have in common is unified inbox, merging of threads, controlling signal-noise.
If thunderbird took that UX approach, had undo send, it'd be indispensable.
There are also small things, like google apps 2FA working out of the box, searching email, that for some reason haven't worked as well for me w/ thunderbird yet. I'm not sure if searching gmail is doing it through their API, or by downloading all mail, but I've had issues with that.