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."
For more info:
Problems after Updating Firefox
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Posted by ellen at September 24, 2007 04:58 PM
this solution does not works for me
version firefox 2.0.0.11 on linux
ff recreates the files removed but no change on menu
worked for me. thanks!
In my experience, you have to also delete the "prefs.js" file. Make a backup of this file before you delete it though, just in case any of your important preferences are lost. When you launch Firefox again a new prefs.js file will be created and it will reset preferences such as your default home page(s).
Your bookmarks, cookies, stored passwords, and the like should not be affected.
This is what worked for me. Until I did this the Add-ons window would open every time I launched Firefox and what's worse: it would be empty! I couldn't manage my plugins at all. Deleting prefs.js fixed it.