The latest release of Firefox contains a bug that hides some of your installed Add-Ons from the Add-Ons menu. This handy extension from Mozilla will bring them back.
Mozilla's working on a more permanent fix at the moment, but if you've noticed some add-ons have gone missing, don't worry — they're still there, they're just hidden from the menu. The Add-On Recovery Tool from Mozilla will bring them back. Just install the Recovery Tool as you would any other add-on, and restart Firefox when prompted. When Firefox restarts, the add-on will remove itself and you should notice that the rest of your add-ons are back in the menu.