There's unfortunately no getting around that maintaining a secure and performant web engine is a highly expensive endeavour. There's a reason why it's just Google, Mozilla, and Apple left (and Apple doesn't even try to implement all the web standards).

If not Google, it'd probably be Bing, or some other extremely wealthy company trying to get something out of Mozilla.

Every time Mozilla has tried to diversify their income, people complain about them trying to make money/commercialise. I mean I don't like it either, but they're in a tough spot.

They're seen as evil bastards if they take money, and they're seen as evil bastards when they try to make money.

At least with Google there's a possible antitrust case if they suddenly pulled funding, given their market position, and that gives Mozilla a bit more leverage than I think they'd get with other deals.