Mozilla takes donations, so it isn't necessarely free. Also Mozilla is wondering why no one wants to use their software, this is one of the reasons why.
This is a wild take. Taking donations doesn't invalidate the fact that it is available to everyone for free. By definition donations are voluntary. No part of Thunderbird is behind paywalls of any sort.
Donations directed to Thunderbird go to Thunderbird, and are 99% of Thunderbird's income.
The income streams are not mixed.