If you've noticed some Firefox Add-ons missing all of the sudden, this is because there's a bug in the latest Firefox update. Your add-ons aren't gone, though, so don't worry. They're still operating — you just can't see them.
While Mozilla puts together a fix for the issue, they have paused Firefox updates. When the bugs are squashed and a patch is ready, updates will resume. So just be patient for now and things should be back to normal in due course.
Issue discovered with Firefox add-on upgrades [Mozilla Blog]