hi, this is/was caused by an unforeseen technical problem unfortunately (an expired certificate used in the process of verifying addon signatures).

a fix for this is already being rolled out - for details, please see: Add-ons disabled or fail to install on Firefox

as you appear to be on an outdated and abandoned version you should probably looking into upgrading to the current firefox 66 first though...

Note, adblocker ultimate has the same error message...

My problems with "updating" is the fact that coders are prone to add/remove/replace/disable/ or otherwise make obsolete some features or plugins at will... this is unacceptable, so I try not to upgrade.

according to some replies from mozilla employees they will look into possibilities for fixing this issue for older firefox versions, but the current focus is (hopefully understandably) on getting this rectified for all users of supported browser versions first...

No surprises there!

It worked the other day, they broke it now " they will look into possibilities for fixing this issue for older firefox versions'

Not a surprise there.

See also:

• /questions/1258208 plugins could not be verified firefox esr 52
• /questions/1258275 How do I re-enable the add-ons WITHOUT being forced to upgrade

No matter, I dumped FF and am now on Opera, I ALWAYS have MS/IEX to use.

For anyone else:

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.