Update: I did a clean re-install by deleting all Mozilla registry keys, all Mozilla folders on C drive, rebooting the machine and all the other tricks I could find. Unfortunately, the issue persists.

John, have you tried manually creating a new profile?

You can create one by entering "about:profiles" into the address bar, selecting "Create a New Profile", and following the instructions.

However, this is unlikely to solve all the add-on issues. The root certificate that Firefox uses to verify add-ons has expired. You can read about this here: https://support.mozilla.org/en-US/kb/root-certificate-expiration

There is no setting you can change in old Firefox that will update the root certificate. This can be achieved by updating to one of the versions listed in the article.

EDIT: Just to note that Firefox Developer Edition does have a setting to skip extension verification, but of course there will be security and possibly other issues in doing this.

Thank you for the info. Well, that explains it.

So apparently the fact that the issues started after playing around with Revert8Plus was just a coincidence.

Firefox is the only browser that won't let me launch a newer version. There is something underneath that seems to be too much for my system to handle. Too bad because it's my favorite browser.

In Developer Edition, you can bypass extension verification, but it's still an EOL insecure version.

Add-on signing in Firefox

To force reverification after the preference change:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste xpi-signature-verification and pause while the list is filtered

(3) Click the trash can at the right end of the row for the app.update.lastUpdateTime.xpi-signature-verification preference to remove this preference

(3) Do a normal exit/restart of Firefox, and within about a minute Firefox should reverify your extensions

@jscher2000 I just entered the first link you provided and found the solution there. Changing "xpinstall.signatures.required" to false fixes the problem.

Thank you!

I normally use Firefox Developer Edition, version 127 because any later version always freezes my whole computer

So a random Beta build of Firefox 127.0 instead of the Firefox 27.0 Release. Note you would not have Extensions disabled if you had Firefox 128.0 or later. I say Beta because the Developer Edition is the same as Beta but with additional developer features. A version on Beta channel can get nine builds in testing as that is what the 127.0b# was about.