This might be something that has to be reported to upstream but I have no means of telling which is why I post it here. When I upgraded from Mozilla Firefox 2.0-r2 to 2.0.0.1, all my add ons disappeared from the add on list. They are still in the extensions.rdf file but will not be displayed in the list under Tools->Add-ons any more. I can install new extensions there which will show up in the list but never get activated. The browser does not seem to register the restart and therefore will keep telling me that a browser restart is needed to activate the add on.
I am unable to confirm this by any means. Suggesting closer as works for me.
marking as WORKSFORME as per anarchy's request.