On May 3, Firefox users all over the world noticed that the browser’s add-ons suddenly stopped working and disappeared from the toolbar. This caused major consternation, as you might imagine. Mozilla has previously made changes to Firefox which disabled some add-ons, so there was initially some concern that this was intentional. However, it turns out that someone at Mozilla failed to renew a critical security certificate, which then expired on May 3rd.
Mozilla added certificate checking to Firefox’s add-ons (extensions, themes, search engines, language packs) some time ago to weed out malicious add-ons and prevent them from being used. When the main certificate expired, Firefox suddenly identified all add-ons as invalid, and disabled them.
Once people started noticing the problem, they naturally tried to find workarounds, some of which did more harm than good. Mozilla scrambled to solve the problem, and on May 4 pushed out an official, temporary workaround using a little-known Firefox feature called Studies. Once installed, this fix did re-enable add-ons for many users, but didn’t help if the Studies feature was disabled, and was only effective for desktop versions of the browser.
On May 5 a new version of Firefox was released by Mozilla. Firefox 66.0.4 includes a single change that fixes the certificate expiry problem. There are a few caveats: some add-ons may need to be re-enabled manually. Certain add-ons will remain disabled. Other add-ons may need to be reconfigured.
This was a major (and embarassing) blunder, but Mozilla handled it reasonably well, although the information they published was occasionally somewhat misleading. There’s a useful record of what happened on this Mozilla blog post.
Update 2019May10: Yesterday, Mozilla published a followup/apology post.