You need to go to the Mozilla Firefox download site and look at the System Requirements page for the version you want to use.
I got one machine that is stuck at Firefox 188.8.131.52 (this one) because it is Win98, and none of the newer versions have support for Win98.
Some Firefox 4 have a problem with blinking GIFs that was really annoying (cairo lib on Linux). I have upgraded my Linux all the way to Firefox 10, which is a bit larger but does not appear much different than 4.
Last edited by selfprogrammed; 02-21-2012 at 11:39 PM.