A FIX IS AVAILABLE TO MOST USERS
Mozilla is rolling out a hot-fix. Official details from the Mozilla blog:https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
It should just start working again for most people with the latest updated version of Firefox.
It uses the "Studies" feature, which is an option in the Security & Privacy section (towards the bottom). It is turned on by default in most installs.
If not, you will need to enable "Studies" in the privacy options and Firefox should automatically pick up the hotfix being pushed out by Mozilla.
You can turn off studies after if you had it turned off.
If you are on an older version of Firefox, and there is no Studies option, you might try upgrading Firefox so that the Studies hotfix can fix the problem.
It may be also possible to manually install the hotfix XPI extension file.https://storage.googleapis.com/moz-fx-normandy-prod-addons/extensions/hotfix-update-xpi-intermediate%40mozilla.com-1.0.2-signed.xpi
Nightly and test builds of Firefox have a configuration setting that can disable extension signature verification. The main production release doesn't.
The reason Mozilla signs addons is so that people can't trick you into installing an extension that can steal passwords from the browser or from the LastPass extension, for example.
They just screwed up by not having the replacement certificate already installed before this happened.