Sorry, you're right, the add-ons are really not corrupt. The problem is, a critical certificate used in verifying add-on signatures expired. So everything fails testing.

Mozilla is distributing a hotfix to re-enable using, installing and updating add-ons. Please check this article to make sure your Firefox is set up to receive the patch(es):

Add-ons disabled or fail to install on Firefox

caveroy1 said

There was a Bug-A-Boo with a Certificate Expiration Date dealing with the Add-ons. There's a new FF Version with the main Fix, 66.0.4.

Mozilla Blog Post Update:
A Firefox release has been pushed — version 66.0.4 on Desktop and Android, and version 60.6.2 for ESR. This release repairs the certificate chain to re-enable web extensions, themes, search engines, and language packs that had been disabled (Bug 1549061).
There are remaining issues that we are actively working to resolve, but we wanted to get this fix out before Monday to lessen the impact of disabled add-ons before the start of the week. More information about the remaining issues can be found by clicking on the links to the release notes above.
(May 5, 16:25 EDT)

~Pj