You can download them from Mozilla website, then build the RPMs by yourself. See this site: http://fedoranews.org/tchung/
. This is specially needed for localized FF versions (for example, i had to build my RPMs for Firefox and Thunderbird in spanish, since no site have updated RPMs in languages other than english)
Also, if you want to update FF1.5 (using the now-working autoupdate), you must run FF as root for do the update. Let it download the updates, restart for install them, and you're ready!