Mozilla is a lost cause, and Firefox is too big to be rescued by one singular web-browsing development entity (that's because the web has become too complicated so as to allow for terminal capitalism).

The solution is, honestly, for a well-constituted organization to hard-fork the web standards model as a whole package, to start a sort of "re-web" project. Pick up something like gemini but iterate it to a level where it can actually be useful (gemini, nifty as it is, is actually less capable of layout and styling than the printing press from the 1500s and that's saying something). Maintain a "convertible subset" of HTML+CSS as an input option for people who want to maintain and develop simple sites that might want to eventually convert to re-web.