Top posting and not quoting anything as I'm coming from a different POV. If the OP needs firefox 91 (or whatever), there is another option, installing the package available from Mozilla as a separate executable.
(Aside: I had to do that (for an earlier version of Firefox) because a website that I must access periodically changed their design / minimum browser requirement (without publicizing it very well).) I don't remember exactly what I had to do, but the instructions (and downloads) are available and reasonably easy to find on the Mozilla website. Now I have two versions of Firefox installed (on my Jessie system (I know, I know, I still plan to install bookworm on a new (to me) system -- life seems to keep getting in the way). If I simply start Firefox in the normal way (I type "Firefox" in the <alt><F2> textbox, I get the "current" version of Firefox installed in Jessie. When I need the newer Firefox, I type /opt/firefox/firefox in the same <alt><F2> textbox. IIUC, it is sort of a self-contained binary executable that doesn't depend on libraries or such from the "main" Debian Jessie system. (I didn't notice that anyone else had suggested this, so I did -- sorry if it is a duplicate suggestion (or of no value).)
