You can't effectively paywall it because not only is it open source, but there are many nearly equivalent competitors all of which are free. Any subscribers would essentially be donors.
There are people like yourself who would be happy to donate, but not nearly enough. Replacing MoCo's current revenue with donors would require donations at the level of Doctors without Borders, American Cancer Society, or the Make-a-Wish Foundation.
Turning into one of the largest charities in America overnight simply isn't realistic. A drastic downsizing to subsist on donor revenue also isn't wise when Mozilla already has to compete with a smaller team. And "Ladybird does it" isn't a real argument until and unless it graduates from cool project to usable and competitive browser.
Thunderbird has succeeded at doing this and is in a somewhat similar spot (though huge asterisk there given the existence of Chrome)
> You can't effectively paywall it because not only is it open source, but there are many nearly equivalent competitors all of which are free.
You're forgetting that people will buy a product on brand identity alone. If the Firefox brand is solid enough, those forks won't matter.
Oh no, it would be a donation and it's not going to completely replace all the funding of the parent entity of the project mentioned, therefore it's not realistic or worth trying. Right... That's a lot of arguments unrelated to what I wrote.