The problem began earlier in the week when the browser suddenly disabled all extensions and prevented users from installing new ones. Mozilla has since revealed that its engineers forgot to renew an intermediate signing certificate. As Firefox doesn't allow unsigned extensions, they were disabled for everyone.
Firefox's developers responded extremely quickly, pushing fixes through Nightly rings and releasing the hotfix to all via an optional ‘Studies' channel. The general fix does not use the studies system and should have fewer issues.
However, it's worth noting that some addons may still appear as unsupported in Firefox 66.0.4. In this case, users should be able to reinstall the addons without any data loss. Themes may also need to be re-enabled manually, while homepage preferences customized by an addon will be reset to default.
“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,” said Mozilla. “More information about the remaining issues can be found by clicking on the links to the release notes above.”
Users have been understandably frustrated by the issue, which seems to have occurred through an obvious oversight. The requirement for extensions to be signed was always a slightly controversial move, and it's come back to harm the browser in this case.
Back in February, Firefox 65 caused HTTPS filtering issues on Windows PCs with Avast and AVG. Since then, Mozilla has debuted a number of features, including Windows Hello support and anti-cryptomining measures.