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Mozilla is aware of a bug that disables all Firefox add-ons (Now fixed)
[windowslatest.com] Update: Mozilla has identified the problem and issued a fix for Firefox, Firefox Beta and Nightly. The update will be automatically applied in the background within the next few hours and bring back extensions to the original state. There is no action required by the users.

"Our team has identified and rolled-out a fix for all Firefox Desktop users on Release, Beta and Nightly. The fix will be automatically applied in the background within the next few hours. No active steps need to be taken to make add-ons work again," the company said in a new blog post.

Mozilla is using Firefox's Studies system to deliver the fix on short notice and the general fix is also being worked on. "We are working on a general fix that doesn't use the Studies system and will keep this blog post updated accordingly," Mozilla explains.

To turn on the 'Firefox's Studies system', follow the instuction below:
[Techguy.com Forum] Posted by Cookiegal; There's an update to that article and in order to get the update you have to allow "studies" to run in order to receive the fix. This is under Tools - Options - Privacy & Security and scroll down to the section entitled "Firefox Data Collection and Use". I had everything disabled there but that will prevent the fix from coming through. I've temporarily enabled it and will leave it like that for a few hours to see if the fix arrives.

https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
After turning on the above, turn off Firefox and wait a few seconds (to make sure Firefox shut down completely) then restart. This fixed my issue!
Posted by: Seeking Cure For Ignorance 2019-05-04
http://www.rantburg.com/poparticle.php?ID=540250