Mozilla has a budget of around 200 mil for software development, so the 7 mil are probably not enough. Not defending the high pay though.
Also, AI Integration into browsers could very well be a deciding factor for mainstream users when choosing a browser. So having some expertise around e.g. running LLMs privacy preserving on client hardware for page summarisation could pay off. Llamafile for example, is something cool coming from the Mozilla AI stuff.
Well LoL has no official Linux support, so a low current number of users is no indication of the size of the potential Linux player base.