I don't think forum volunteers yet know the cause or remedy for this problem. However, know that dozens of people are reporting the same thing in recent hours, so it may be due to a rolling update gone wrong.

As an interim update: The Add-ons team is working on a fix for this. It looks like a certificate used to sign many popular extensions expired. (The verification process checks that certificate.)

I don't know how soon re-signed versions of those extensions will become available, or whether another workaround will be discovered. We are all hoping to learn more soon.

See also: Bug 1548973 - (armagadd-on-2.0) All extensions disabled due to expiration of intermediate signing cert

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

Skcamero said

Every single one of my Firefox add-ons were disabled due to being 'unsigned' even though all of them have been obtained via the Mozilla store...

Additional Info: From Mozilla Blog:

Updates – Last Updated: 19:28 EDT May 8 2019

We’ve released Firefox 66.0.5 for Desktop and Android, and Firefox ESR 60.6.3, which include the permanent fix for re-enabling add-ons that were disabled starting on May 3rd. The initial, temporary fix that was deployed May 4th through the Studies system is replaced by these updates, and we recommend updating as soon as possible. Users who enabled Studies to receive the temporary fix, and have updated to the permanent fix, can now disable Studies if they desire.

For users who cannot update to the latest version of Firefox or Firefox ESR, we plan to distribute an update that automatically applies the fix to versions 52 through 60. This fix will also be available as a user-installable extension. For anyone still experiencing issues in versions 61 through 65, we plan to distribute a fix through a user-installable extension. These extensions will not require users to enable Studies, and we’ll provide an update when they are available.

"Updates – Last Updated: 11:51 EDT May 8 2019

Firefox 66.0.5 has been released, and we recommend that people update to that version if they continue to experience problems with extensions being disabled. You’ll get an update notification within 24 hours, or you can initiate an update manually.
An update to ESR 60.6.3 is also available as of May 8th. We’re continuing to work on a fix for older versions of Firefox, and will update this post and on social media as we have more information."

~Pj

https://support.mozilla.org/en-US/questions/1258165#answer-1218833 fixed the issue for my v54.

See comment 51 in bug 549604 for a link to install the legacy version of the hotfix extension to install the intermediate certificate.

• hotfix-update-xpi-signing-intermediate-bug-1548973-legacy@mozilla.org-1.1.2-signed.xpi
• https://bugzilla.mozilla.org/show_bug.cgi?id=1549604#c51

There are still unresolved issues that prevent installing the legacy hotfix automatically, but a manual install should work.