The problem with Firefox is that the money has to go through Mozilla, and Mozilla is not spending most of that money on Firefox. You cannot sponsor the development of Firefox directly, so your money ends up being wasted.

Firefox/Gecko also just can’t be as relevant as a Chrome/Blink competitor since Gecko doesn’t support embedding on desktop operating systems, which precludes things like Electron-style wrapping and hybrid apps. It’s a fatal flaw that Mozilla doesn’t seem to have any interest in addressing.