If you asked your engineer peers how they would feel if you were paid an equivalent salary, I am confident some of them would admit that they would not like it. Privately, I think most of them would definitely not like it but would probably refrain from publicly admitting as much.

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.