hi EP1952, please see the banner at the top of the support page - unfortunately there's a technical issue (an expired certificate used for addon verifications) today.

mozilla is already rolling out a hotfix for this problem, using the studies system. you can check if you have studies enabled:

1. Go to [=] > Options > Privacy & Security.
2. Make sure Allow Firefox to install and run studies is check marked.

if you had the preference disabled, reenable it temporarily and restart the browser. after your add-ons have been re-enabled you can disable the setting again.

This Add-on problem seems to be on all 52...66 Firefox versions. You can not install any Add on from web, and not even from your own hard disk.

I found a stupid, old-time work-around to this problem, but at it works and solves this immediately.

Turn your computer clock backwards some days, like May 1 2019 or something.

Then you can immediately re-install all your add-ons again. After I turned the clock back to original Date, the Add-ons still remain and are working.

I have to do some more testing, if the problem will come back. I just wanted to share my findings this far.

I do not know how, but suddenly all add ons are working again.