In any fedora 1-6 distribution version you can find firefox 1.5
and unable to use great features like themes and other addons and many securities issues
here is simple method to upgrade firefox 1.5 to 2.0.0.1

simply type

yum -y -t –enable=development update firefox

if any conflict occur, generally with mozilla-1.7.12

then type ( only if error occurred )

yum remove /usr/lib/mozilla-1.7.12

and then type

yum -y -t –enable=development update firefox