Add-ons are checked every 24 hours, the problem affected users at different times.
Mozilla has issued the patch for a certain certificate issue from its end that disabled all the add-ons. Now, the company is out with a detailed explanation about what the issue actually was and why it took so long.
On the official blog, Firefox Chief Technology Officer Eric Rescorla explained that one of the certificates used to sign add-ons expired just after 1AM UTC on May 4. As add-ons are checked every 24 hours with the time of checking being different for each user, the problem affected users at different times.
He further details how the team first disabled signing of new add-ons and then immediately push a hotfix which suppressed re-validating the signatures on add-ons. Later, the team generated a new, valid certificate and installed it remotely into Firefox.