If you want to advertise, have a newsletter that people can opt into.
If you need money, allow people to pay for just Firefox.
I think many people overestimate how many people would pay for Firefox.
Unfortunately they hired a bunch of incompetent executives who would much rather starve Firefox development while spending all of the money on realizing their big business vision where they acquire useless startups then ruin the browser by turning it into a marketing platform for the aforementioned. Besides bonuses and big salary bumps, of course.