Fedora 8 comes with FireFox 2.x.x out the box. It seems policy of whoever decides these things is that yum should be restricted to the same major version of software as first installed. So it's FireFox 2 only on FC8.
Unless... Some kind sole creates a repo for FC8 that includes FireFox 3.
So.... as root...
wget http://rpms.famillecollet.com/remi-release-8.rpm rpm -Uvh remi-release-8.rpm yum whatprovides firefox
At this point I get a list containing only FireFox 2.x.x versions. Quick check in /etc/yum.repos.d/remi.repo and set enabled=1 instead of 0.
Retry yum whatprovides firefox this time it picks up both ver2 and 3.
Now I can install Firefox 3.x.x from this remi.repo:
yum install firefox-3.0.5-1.fc8.remi.i386
A quick restart and add-on check and we're away, all FF3'd up :)