Answers: If you have a firewall, it may be blocking firefox from installing add-ons. Go into the settings and change it to prompt before blocking.
Extensions.cache, extensions.ini and/or extensions.rdf may be corrupted. Go into your profile folder in Documents and Settings > [User] > Application Data > Mozilla > Firefox and delete those three files.
ok hit F4 on your keyborad see if the sidebar shows up? if it dose there are all you add ons.
when you go to tools do you see any of your add ons
go to tools go to general tab and clck on manage my add ons it should open a sidebar or window and show you your add on. if that don't work message me and i'll talk you though a few more steps.
It could be your security setting, either in control panel or firefox, fiddle around and see if you can fix it.
if u ave problems with firefox just switch back to good old IE BTW i hate IE and firefox works good for me