It probably doesn't help that you can't choose to donate to/for Firefox. If you disagree with other things Mozilla is doing, perhaps moreso than the things you agree with, then you don't really have much options.
For many people, advertisements - especially ones enabled without announcement, sneakily - are simply an unacceptable red line, nomatter how important Mozilla thinks they are.
Right, now the goalposts have shifted. You aren’t donating because “you can’t donate specifically to Firefox, only to Mozilla as a whole”. What ridiculous excuse will you people come up with next?
Alright, I'll bite. I donate quite a bit of my income to FOSS projects (which is pretty low due to living in the third world, so it's not a lot of money). I would gladly donate specifically towards Firefox development, but they are not spending donations on Firefox at all, and I would rather spend that cast (which I don't have a lot of) towards some other technical goal like Wayland infrastructure development, instead of wasting it (as I see it) to advance some social goal in the US which is completely alien to me.
I should also add that I never disable any telemetry in Firefox or anything that helps them earn a cent or two (as long as it does not abuse my privacy too much, and there were not a lot of those during 15+ years of using Firefox exclusively.)
The problem is not that your donations will go to other things but including Firefox. The problem is that there is no way for the money of your donations to end up in funding Firefox development. Mozilla Foundation money does not flow towards Firefox corporation.
Note, I've heard that Mozilla takes donations, but I don't think Firefox does.
Personally I don't mind at all if Mozilla wants to display ads in Treeherder or in their corporate offices, I wouldn't even feel like I want to pay to remove those :)
My point being, I don't expect firefox donations would amount to very many millions, but it's important to note that today the option doesn't really even exist.
Guess what, “the people who want to make the software better” can start today! Just fork it, it’s all open source! Wait, why hasn’t that happened yet? Is it because those people have bills to pay? Hmm...