Despite what others keep saying, this add-on was NOT disabled because of a Firefox update. The problem is that they bizarrely (how it happened hasn't been revealed yet as of 4May2019-13:00) let a signing certificate expire.

This in turn caused ALL add-ons to be disabled because when they were checked for compliance with Firefox's policies last night at midnight they were ALL flagged as "unauthorized" (or "non-compliant") add-ons.

A fix has been issued. Simply go to Tools (press the Alt button to see the menu at the top of the browser) > Options > Privacy & Security and scroll down to 'Firefox Data Collection and Use' and make sure a checkmark is in 'Allow Firefox to send technical and interaction data to Mozilla' which will un-gray 'Allow Firefox to install and run studies' which should then also have a checkmark.

Close and re-open Firefox. To see if the Hotfix got applied, go back to same section as before and click 'View Firefox studies'. You should see an entry about 'hotfix-update-xpi-signing-intermediate-bug-1548973•Active'.

Your add-ons should now be re-enabled and working.