Have you tried opening firefox, going to the help menu, and selecting "check for updates"?
Assuming you have this option, it should download and install the latest version for you. (here's hoping!)
Have you tried updating it from sources? (i.e. downloading sources from mozilla website?)
Also, have you tried using the "bundled" update manager, other than easy-urpmi? (as to what package that would be, I know not, sorry)
Hope this helps!