You can downgrade back to the good version here: https://support.mozilla.org/en-US/kb/install-older-version-firefox?redirectslug=install-older-version-of-firefox&redirectlocale=en-US Just make sure to IMMEDIATELY go into the settings and turn off auto-update.

@Greg, Thanks for responding. I marked your response as a solution because # It answers the question I posed. # The e-mail notification I received doesn't provide a link to the thread, but only links to accept or reject the answer as a solution. Unfortunately, it doesn't really solve my problem: * Unless Mozilla listens to the many complaints about the new Proton GUI, I am just postponing the inevitable. In order to postpone it as long as possible, I plan to [[Switch to Firefox Extended Support Release (ESR) for personal use|switch to the ESR version]] switch to the ESR version. Since it is much older (78 instead of 88) I can only hope that I don't run into plugin incompatibilities. * I have my bookmarks stored in Firefox sync, so I will presumably not lose them when I downgrade, but according to "[https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation#w_what-happens-to-my-profile-if-i-downgrade-to-a-previous-version-of-firefox What happens to my profile if I downgrade to a previous version of Firefox?]" I will lose my history and perhaps other data as well; the article isn't very detailed. I find the history very useful and have a few years of it. I would also like to know what other data I will lose with a new profile. I will search for (or start) (an)other thread(s) on these issues.