Yeap. It's mentioned in their financial reports that user donations represent more than 99.9% of our annual revenue[0]. Also seems their staff is mainly engineers/developers, and all the expenses are concentrated to their product*. Thunderbird doing what Firefox should.

[0]: https://blog.thunderbird.net/2023/05/thunderbird-is-thriving...

*Though Thunderbird is Gecko-based so can be said in part, perhaps a significant one, they're depending on Firefox development.

It doesn't matter if they are or not really.

As of right now Thunderbird doesn't make any money, it relies on 'Donations' which isn't at all sustainable.

I can see Thunderbird is planning to do a pro plan, but it is behind a waitlist so the total sum of revenue Thunderbird is making relative to Google's $500M deal is close to zero.