Firefox extensions and add-ons disappear

After a recent update to Firefox, all my precious debugging extensions disappeared. The Add-on’s window showed nothing installed. This post on the MozillaZine Forums explained how to force Firefox to rescan your addons and get them all back:

“Crikey… My extensions have gone or are unusable….
The update has confused the extension manager…. This procedure will reset it, so Firefox rescans your collection of addons, to see what is really there.

Look in your Profile Folder (see above for how to find it) and Delete these three files
extensions.ini
extensions.cache
extensions.rdf
Make sure Firefox is not running when deleting these files. Firefox will regenerate these files on restart.”