I just posted this. All my addons have been moved to Legacy Extensions too. And I see about a couple dozen people have posted the same problem in the past hour.

Only thousands of people, I think.

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.

There is no fix yet, in spite of a Mozilla rep saying one's being applied.

jrenfroe said

There is no fix yet, in spite of a Mozilla rep saying one's being applied.

Go to the 3-bar menu => Help => About Firefox and see if you're ready to update to Firefox version 66.0.4 yet.

See : https://www.mozilla.org/en-US/firefox/66.0.4/releasenotes/?utm_source=firefox-browser&utm_medium=firefox-browser&utm_campaign=whatsnew

And also : https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

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

jrenfroe said

There is no fix yet, in spite of a Mozilla rep saying one's being applied.

Please mark one of the answers here that best fits as 'Solved'.

Thank you,

~Pj

I still have this issue with Last Pass

Hi jj--, could you start a new question and provide your system info along with your post? This is the link to get started, please keep scrolling down past suggestions to continue with parts 2 and 3 of the form:

https://support.mozilla.org/questions/new/desktop/customize

Also, does Firefox display an explanatory message with the extension as to why it cannot be re-enabled? Please include the specific wording in your new post. Thanks.

Upon review my issue may be different.

Last Pass is not working on certain logins, Mozilla being one of them. LP recognizes the site but is unable to fill in the login. No message displays, just a blank even when I manually select "fill" which is usually automatic. I will post a new question as suggested.