In general I agree: Open source projects are super hard to monetize and too much work does not get donations, flowers or even thanks.
For Firefox specifically I am not so sure, especially when Thunderbird seems to be doing good with their donation based model.
As long as Firefox is run by Mozilla throwing millions at their incompetent leadership, I will not donate a cent to Firefox.
If Firefox would get forked by some developers I’ll happily donate money to them and given Firefox high visibility/importance, this might work out, like Thunderbird did.