You can't walk away from your day job at $BIGCO, start your own company and expect to get paid like you used to, it doesn't work like that (at least at the beginning).
Your pay is having a large % ownership in (what you think will be) a valuable company down the line.
My issue, I guess, is not with salary, but moreso the value add. 75k does not seem like enough to A.) Live on, really, B.) Extend runway, really, C.) "Fix the internet", really. I can understand trading in cash for agency, I would absolutely do it. But this doesn't really seem like that either, given that presumably lots of founders would need to either seek further funding or work second jobs.
It's also not just the money. It's credibility by being invested in mozilla (and an investor who is invested in you having more successful funding rounds), it's the marketing and hype that come with it, it's the opportunities to employ people, it's the connections that come from being acquainted with investors and a tech giant, etc.