I won't comment on whether I felt I was underpaid in my years at Mozilla (though I'll say I do make more now, at a for-profit company), but it is at least worth pointing out that Mozilla's bonus system extended to everyone. And until early 2015 it was based on company-wide performance, so even if you had a project that was languishing you could still get a good bonus if the company as a whole was doing well (in 2015 it switched to a significant chunk being based on individual performance and metrics, and I wasn't there long enough afterward to have a worthwhile opinion on how that change turned out).