Can't be sure of course. But there are at least some forks of FF that devs of FF could keep doing work on if Mozilla just stopped FF. I have been daily driving Zen-browser for at least six or eight months. It is very good as far as FF forks go, and if folks that do the work for the core browser things that aren't UI/UX were to jump-in. It could be able to keep moving forward as far as standards and security updates are concerned.

The only major issue (aside from the above) would be setting up an extension "store" since Zen does just point to FF's "store." Which is an issue for basically all the Chromium-based forks that aren't Edge and Opera to my knowledge. I hate that those forks still lose things like uBO even if they still support it instead of the V3 stuff.

Those are very much not trivial things and are honestly what would kill all of FF and the forks if not able to happen. Funding would be the first thing needed for devs to keep going if their main income is from their work for Mozilla. It would be at least easier than completely starting from zero.