Shoutbox

were you able to get extensions before? or has this problem been occuring since the first instalation of firefox?

Edit: because i'm thinking this:
if you had pervious extensions, did you update them? - this could be causing the problem

did you uninstall them prior to reinstalling firefox? - maybe one of the previous extensions is screwing something up..

also, this is a long shot but maybe if you have a firewall it is blocking firefox from installing correctly