I think one of the recent upgrades to firefox-2  caused a strange
failure of all my add-ons: the list showed many of them were disabled,
and the rest just didn't work. (Ads, arrgh!) I found the following
in the error console:
OldInstallLocation has no properties
No chrome package registered for chrome://navigator/locale/navigator.properties .
No chrome package registered for chrome://adblockplus/skin/adblockplus/png .
and so on. Searching around, I found a similar problem  with this
Try deleting these files from your profile:
which worked, although I didn't have an extentions.ini file in my
profile directory. The strange thing is that this happened on my home
computer but not my work one, even though they both run Ubuntu and
have a similar range of Firefox add-ons installed. It was quite
frustrating until I found it --- does anyone know why this happened,
and how to prevent it in the future?
(Also, any idea when Firefox 3 will handle a good range of add-ons?)
 I'm not currently using firefox 3 because it wiped out my add-ons
even more thoroughly; after I'd run it and tried firefox-2 again, they
were still disabled; I had to delete my ~/.mozilla directory and
restore it from a daily back-up to get everything to work.