At the same time, even a tiny bit of friction is enough to get people over the mental hump of paying for something.

They could easily gate off certain features behind a paid build, so either you pay or compile it yourself from source. Downstream packagers could of course do whatever they want (eg Debian). However, it creates a minor amount of friction for a relatively large fraction of the user base, and moreover sets the baseline expectation that this is not really "free as in beer", even though it remains "free as in freedom".

See also: Sublime Text, which, despite being closed-source, is 100% free-as-in-beer to use in perpetuity, and yet somehow they make enough money To not only continue development, but even start developing other products (Sublime Merge), even as their brand recognition wanes and their competitive advantage shrinks.

It doesn't have to pay for the entire Mozilla organization, it just has to bring in more money than the random other stuff they've tried. That's not a very high bar to cross.

Wikipedia has mountains of $$$ to back up that a lot of people do pay, even when it's not necessary at all.

https://en.wikipedia.org/wiki/User:Guy_Macon/Wikipedia_has_C...