era un bug
ho risolto:

da http://www.ubuntuforums.org/showthre...hlight=firefox

if someone is still suffering from the problem that they can't visit firefox addons page here's the solution
go to about:config
search for general.useragent.vendorSub
change it from 1.0 to 1.0.4
and it should work

source : https://bugzilla.ubuntu.com/show_bug.cgi?id=10681