Unable to install Firefox addons

[#1]

Have been unable to install Xmarks (formerly Foxmarks) and Reminderfox in Firefox.
The download completes, a prompt appears to restart FF and then when it reboots there are no add ons there.
Does anyone else have this problem or anyone know of a fix please?

[#2]

xmarks went on FF 3.0.15 fine here on vista. my ff is prob out of date as i dont use it for much

[#3]

Since posting I have uninstalled and reinstalled and somehow Reminderfox appeared although I used Revo to uninstall - clearly bits were left.
Anyway, tried again with Xmarks and when it asked to restart I didn't - closed the browser myself/restarted and all was there so problem solved.