Different PC than what you used to start this thread?

No, same PC.

From blog.mozilla.org: "A Firefox release has been pushed — version 66.0.4 on Desktop and Android, and version 60.6.2 for ESR. This release repairs the certificate chain to re-enable web extensions, themes, search engines, and language packs that had been disabled (Bug 1549061). There are remaining issues that we are actively working to resolve, but we wanted to get this fix out before Monday to lessen the impact of disabled add-ons before the start of the week. More information about the remaining issues can be found by clicking on the links to the release notes above. (May 5, 16:25 EDT)" There have been no release notes yet, but ad-blockers may be one of the "remaining issues"?

The internet (and Mozilla) thinks that you are running Firefox 52.0 on WinXP, so you either have a messed up useragent or you're running in Windows XP compatibility mode.

Right-click the desktop icon for Firefox and look at the Properties > Compatibility tab. At the top makes sure that Run this program in compatibility mode for: ..... is not selected.

Then see this support article: https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

The compatibility tab change to blank didn't work (it already was blank; incidentally there was no XP choice). I am running XP and did have version 52 until I downloaded this newest ESR version 60.8.2

I am reluctant to mess with the reset default user agent, until Firefox is done fiddling with updates, as the blog suggested.

Incidentally, the ad blocker I want to restore is ghostery.

For the record, I checked on this computer, and I apparently am still running the Firefox version 52.9.0 ESR (x86 en-US). Not the 60.6.2 ESR. What gives?

The last version compatible with Windows XP is Firefox 52 ESR. See: Firefox has ended support for Windows XP and Vista.

What you need is a new intermediate certificate that will allow proper add-on signature verification. Up to now, support forum volunteers have provided two methods for installing the certificate, both unofficial, listed in this thread from yesterday:

https://support.mozilla.org/questions/1258165#answer-1218833

We don't know whether the certificate will be made available for download from an official site but, if it is, then you could use that link to install from.

firefoxuser1000 said

For the record, I checked on this computer, and I apparently am still running the Firefox version 52.9.0 ESR (x86 en-US). Not the 60.6.2 ESR. What gives?

Firefox 53.0 (technically the 52.0 Release also) and later dropped support of the old EOL WinXP and Vista so the Firefox 60 ESR including 60.6.2esr will not work. The 52.9.0esr was the last as said.

Besides upgrading to Windows 7 or later you could use a light 32-bit Linux distro with a light desktop like Xfce in order to use current versions of Firefox.

Go to about:config

Search for xpinstall.signatures.require

Double click it to set to false.

Wait a few seconds. Your add-on's should come back to life.

My issue was somewhat resolved using

xpinstall.signatures.require = true and setting it to false.

However, now that the disabled add-on's are re-enabled, each of them is highlighted in yellow with an unsupported notification.

OnThePike said

Go to about:config Search for xpinstall.signatures.require Double click it to set to false.

This introduces the risk of rogue updates, so you should consider also disabling extension auto-updates with this setting so you'll have the opportunity to inspect and reject any questionable extension updates:

extensions.update.autoUpdateDefault => false

With that setting, when an update is detected, a new category will appear on the Add-ons page listing available updates.

Mozilla broke my ghostery add-on, Mozilla should fix it. Ghostery, if you are listening, tell Mozilla to make your product available to all Mozilla Firefox users, including us XP hold outs.

firefoxuser1000 said

Incidentally, the ad blocker I want to restore is ghostery.

I want ghostery back too. I don't think it is an XP issue though, I am running 10 and it's gone. I am not positive but I think the problem may now be on Ghostery's end since Mozilla needs to sign off on the extensions and it won't do that if it doesn't meet certain criteria ( something they rolled out along with this new way a signing extensions). I have a support ticket open with Ghostery to look into that.