When does the support for Windows 7 end? Will FX 91 / 91 ESR be the last
version that supports it?
I have not heard anyone say 91 would be the end, and if the end was that close there would have been plenty of mentions and announcements (on this list and elsewhere). I have no special knowledge, but looking at https://data.firefox.com/dashboard/hardware#operating-system I'd guess not for a while yet.