If you are using fedora, have you tried their update service, up2date? you should be able to update that way. I think you can also just type:
yum check update
then locate firefox and type yum install (full file name here)
Or if you are using fedora you can get the Fedora i386 RPM build of Firefox 1.5-4 here:
Those are my suggestions for the easiest way to update firefox, I'd try the fedora update service because I used Fedora for a short while and I know that Firefox showed up as an update there.