Did you know that I can still use a web browser from the last decade to surf websites? Yup.
Even if FF dies, the codebase is opensource and continue to be developed.
[0] https://gitlab.com/groups/librewolf-community/browser/-/grou...
Look at the following browsers: Otter Browser Min Qutebrowser Links Basalisk Surf Bromite (only has 7 contributers)
It is possible. Of course it's possible. Just depends if that situation were ever to occur, whether people would become passionate about that project
Basilisk and its sibling Pale Moon are an example of how hard a fork is (both being pre-Quantum Firefox); features removed because cannot be maintained, incompatibilities, poor performance and many vulnerabilities.