My assumption would be that it's very little, given that Thunderbird was separated out of the Mozilla Corporation to MOZLA (or whatever it's called).

On the bright side, that actually makes me a bit keener about donating to it; donating to the Mozilla Corporation seems entirely pointless given donations make up ~2.5% of their income, and less than 10% of what they spend just on manager salaries, whereas giving it to Thunderbird might actually have a positive impact.

I'm not sure which part it is in their accounts, but their Form 990 says:

https://stateof.mozilla.org/pdf/Mozilla%20Foundation_Form_99...

> MZLA TECHNOLOGIES CORPORATION share of total income: $10,760,074

So they don't break it down, but around 10 million went to the corporation that runs Thunderbird and other projects (versus 658 million to the one that runs the browser)