Firefox browser "armagadd-on-2.0" bug fix
Public Domain — copyright waived
If you are using the Firefox browser, and you have any add-ons installed, they all got disabled at, or soon after, 0000 UTC today (5 PM PDT Friday May 3 2019).
This is a Firefox bug. They forgot to renew a security certificate, so all add-ons suddenly appeared to be unsigned and insecure.
There is a fix available. DON'T try to delete and re-install all your add-ons, because this will probably delete all the settings too. If you're using the desktop version of Firefox, the official fix is described here:
Update Regarding Add-ons in Firefox
(This won't work in Firefox for Android, Firefox ESR, or some Linux Firefox releases. See below for what to do in these cases.)
The official fix involves letting Firefox auto-install what they call a "study". If you got the desktop version of Firefox directly from Firefox, and if you didn't change your privacy settings, "studies" are already enabled, and all you need to do is leave Firefox on and wait up to 6 hours for the fix to be auto-installed. See the above article for how to make sure "studies" are enabled.
If you're using some Linux builds of Firefox, the "studies" option may be grayed-out with the note "Data reporting is disabled for this build configuration". In this case, you'll need to download/install the fix manually. (This also works if you don't want to enable "studies" for privacy reasons, or don't want to wait up to 6 hours for the auto-install.)
Click the following link from desktop Firefox, and the hotfix will be installed:
Note: THERE IS NO OFFICIAL FIX YET FOR FIREFOX FOR ANDROID. I saw one person report that the above download did work on Android, but I can't recommend trying it unless you're brave.
Also, the fix fails to work for a small percentage of people, and they're still working on figuring out why.
Update 2019-05-05: New Firefox versions released:
From the official announcement page, Add-ons disabled or failing to install in Firefox:
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)