I see a difference between "needing money" and "getting motivated by money".
If Ruben was motivated by money I believe he would not have started Trisquel
in the first place. The probability he will make more money with Trisquel
than with another job exploiting his technical skills is very low. The same
can probably be written about Mozilla's employees.
But, again, I do not argue that people working with free software deserve a
remuneration at the level of their contribution. Citing myself:
Nobody here pretends that free software developers should never be paid for
their contributions. Stallman does not say that either. Those contributions
are positive whereas proprietary software developments are negative
contributions from rms' freedom point of view. The former should be
encouraged (e.g., with monetary rewards) and the latter discouraged.
As for your point about donation, I do not see much difference with giving
money to a company. You do not control either what the company does with the
benefits it makes on your purchases.