Mozilla is a no-profit foundation, not a company which needs to be sustainable or be profitable.
I agree Mozilla lost its way but I would still hope in them improving over time than trusting yet another for-profit to serve us in the long-term.
NPOs still need to be financially sustainable/viable. They still need to pay their employees and pay their vendors.
Sure, but this sort of thing (email, plus likely mostly shitty calendaring and contacts) is a very ok business. The fastmail people make a fine living at it (their product is as good as anything outside gmail. If you haven't, you should try it! I'm a happy decade-long customer). But it's not the sort of business that supports the massive employee count that Mozilla has.
I might be misunderstanding the org chart but Thunderbird is operated by MZLA Technologies Corporation, which is for-profit (although I guess it's owned by the non profit Mozilla, similar to how openai was?)