Mozilla overall pays less than your FAANGs or unicorns. Mozilla is in a pretty unique position here, and so I think doing so makes sense.
Which to me is the crux of the matter. If I was your manager at Mozilla and looking to maintain team cohesion and minimize talent loss, engineers would always come first. In my experience it is very common for managers with an engineering background to think that anyone can be a technical writer. Same for evangelizers. People do it for free if they like the technology that they are using. These are not strong bargaining foundations.