Yesterday I had 7 extensions installed in Firefox 15.01. Today they are gone, although I see the in the profile folder. The extensions folder is empty. I'm really steamed, because some of these I rely on heavily. What happened?
If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available.
If this hasn't helped then also try to delete the addons.sqlite file.