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.