I work at a school in NYC and all of our computers have this issue with Firefox. In my case, our Proxy is blocking the update server (probably to reduce the masses of computers from updating all the time). I have to manually download the newest Firefox and install it on each station - never uninstalling. This will "upgrade" the current version of Firefox without removing the current installation or the data on the current version. I am yet to find a solution that will allow me to configure all my workstations to look to our server for the update - and then just manually update one place instead of 400! It does not seem to help that I am in an environment with blended technology 50% Macs / 50% PC.