You can't reasonably claim "they would not pay for the browser, or pay a subscription [fee]" when it is not even possible for a user to donate to the Firefox project specifically.
The only thing that seems to somewhat work is Patreon, which seems to work fine for some developers that are good marketers, but even there the number of creators that can support themselves is very small, and I don't know of any Patreons that support more than a single person.
To support a browser, you need a team, and there is no plausible way to pay for that team with donations.
It's not that nobody has tried financing open source with donations, it's just that nobody has found a way to make it work yet.
No, AFAIK you cannot.
The inability to donate to Firefox is a valid and longstanding criticism of the Mozilla corporate/foundation setup.
Mentioning this deficit in a comment chain about the projects financial sustainability is relevant and appropriate.
It is also true that many projects have funding problems, but that does not negate the parents point.
Mozilla is not a particularly good steward of Firefox and there should be a way to donate specifically to Firefox development, if just for Mozilla as an indicator on what should be their priorities.
What I am saying is that it is extremely unlikely that people would donate to Firefox, even if was possible to do so. At least not enough to actually pay for more than a few developers. (And you need more than a few developers for a browser, even if you cut useless features nobody asked for like Pocket integration.)