@gabriel I don't really have the best answer that can apply to all open-source browsers out there; I don't think it can be universal. One thing's for sure though: if they want to rely entirely on donations, that's totally cool! If they want to rely entirely on partnership deals and profits from a corporation, that's fine too, just make it absolutely clear that you're doing that (so no using the exact same name preceding "Corporation" and "Foundation" please).
I can tell you about one browser called #PaleMoon whose monetary model I know well and is simple: development benefits from both donations and a search partnership deal with #DuckDuckGo. However they're able to do this because they don't have to spin-off a corporation to take the DDG money (which probably just number around some thousands of dollars or maybe even just some hundreds, all I know is that donations still make up most of the revenue). And most of the time it isn't really money that is holding back Pale Moon's development, but lack of developers who can work in the CSS and JS engines.
So that's why it's not simple to answer that question. But if you really want an answer, I guess just accept both kinds of money and try to make it legal? Not sure how Mozilla could do that though... And it's not simple to let the browser's development just be entirely funded by donations either, because while there is no doubt a dedicated amount of users who will give out money, I am skeptical it can be enough to sustain the speed of development Firefox is in right now with its 4-week release cycle.