Get money from donors. Wikipedia shows how that can be done. Or get money from the EU.
Mozilla is a strawman for google that they can claim there exists another browser that is not chrome because of antitrust laws. And now that Microsoft forcefeeds win11 user with Edge it will not take long and google doesnt need firefox anymore.
For sure I would give a donation to firefox if they would build a decent browser which listens to the user but not as they do now.
just my 2 ct
There a nice Mozilla Manifesto explaining the Mozilla mission and their values [1].
Apparently, it's difficult to find a stable, independent, and effective revenue model for an open-source browser that is completely free as a product, should not contain shady ads or product promotions, does not sell user data, and always puts the user first.
This is especially difficult because there are other free browsers with a similar mission that don't need to incur the cost of developing their own web engine.
[1]: https://www.mozilla.org/en-US/about/manifesto/