I was really annoyed when I noticed that Mozilla removed the non-Proton GUI starting with 91.0.
This is especially stupid because 91.0 is an ESR release.
Why didn't they wait for 92.0?!
Well anyway, I took matters into my own hands, and I simply reverted the commits that removed the non-proton code.
Now, I'm not a coder and it's possible that I messed up something while I was using mercurial, but in a first test, it seems to work.
Proceed at your own risk though, maybe I removed/added too much or too little, and maybe there are security implications which I'm not aware of.
So here is the patch: restore-non-proton-v2.patch · master · torvic9 / Firefox · GitLab
You will also need to restore a missing PNG file: tab-overflow-indicator.png · master · torvic9 / Firefox · GitLab
which has to go into
(a binary diff did not work here, that's why it's not included in the patch above)