EDIT: harlanji's response, below, is the much better one IMHO. I let the parent frame it as a question of compensation. As harlanji points out, that was the wrong question.
And even then, half of the work is still done for free.
And that's only for the most famous one. E.G: the huge majority of free libs in JS or Python are purely pro bono.
And even after the money arrived, some very popular software have very little money compared to the produced work: eg: the PSF.
All in all, my entire computer contains millions of euros of software I didn't pay.
It's sad enough you don't want to participate, but it's a choice we must respect. But don't be discouraging other people actually giving something, it's insulting.
As a student, and someone who wants to become a strong engineer over the next ten years... this kind of opportunity is fantastic. You can see it as "uncompensated labor", I see it as a way to learn, get my code "out there", and get better at my trade.
Open Source never really cared so much about its contribooters as they cared about open source. If stallmann would start worrying about its contribooters beeing able to live decent lifes with familys, without having a boss breathing down there neck- maybee then.
But it guess that is just not that sort of freedom thats high on the priortiy list. Open Source as a lifestyle is currently limited to students and beginners.
I would consider this as contribution to this wider community beyond just Mozilla's bottom line.