Why not just use Firefox until that hypothetical point comes, then switch to another if there still exists one?
Of course, there are ways to continue after end of support, you can stay on the same version and stop updating, which is unconvenient in some websites and might block some, but if many people do it, then websites won't be able to safely rely on the features being available on their users browsers.
You can also fork it or use a fork someone made, but browser dev is a lot of work (in the engine mostly) and disagreements.
Why not just use Firefox until that hypothetical point comes, then switch to another if there still exists one?
Of course, there are ways to continue after end of support, you can stay on the same version and stop updating, which is unconvenient in some websites and might block some, but if many people do it, then websites won't be able to safely rely on the features being available on their users browsers.
You can also fork it or use a fork someone made, but browser dev is a lot of work (in the engine mostly) and disagreements.