I installed Ubuntu 9.10 with installation disc in my 4 computers plus I had the similar issue. Firefox cannot install addons. I downloaded "Xmarks" along with "Webmail Notifier" from their websites, free from Mozilla site as well as I could set up them without problem. Then I copied the .mozilla directory to the other computers and problem solved. But if I like to set up one more Addon still it's a problem. I feel that is a Mozilla problem intensified by the server issue we have in. (to update Ubuntu we have to vary to the principal server).