I have extensions and use them every day, but when I look using tools>add-ons>extensions there is nothing there. If I click on appearance or add-ons I see something, but extensions just gives a blank screen. Oh, get add-ons simply says 'loading'.
I know my extensions are there, but why can't I see them? I used to be able to. I use XP and FireFox 17 (I think).
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.