On one hand, if people are happy to do work for free, a company would be stupid not to make use of it. On the other hand, governments should probably outlaw it since it really is just a loophole to pay less than minimum wage. ("Volunteer for 2 years for free, and then we'll give you a minimum wage paid position" = "On average over your career, you get less than minimum wage")
https://www.theguardian.com/media/2019/aug/07/guardian-broke...
And one of the claims / complaints of TFA is that Mozilla had a massive income for many years, and could have built up an endowment; but instead chose to live "hand-to-mouth", spending their income on useless failed projects that have nothing to do with Firefox. (And CEO pay of course.)
Not arguing against the overall point but Thunderbird has nothing to do with Firefox either and is still a thing Mozilla's reason for existing is not just their browser.