rpm -ivh http://www.leemhuis.info/files/repo/fc6-thl-ff2/6/i386/thl-ff2-for-fc6-release-1-1.fc6.thl.ff2.noarch.rpmor something similar to install the repo file and the key which was used to sign the packages. Then run
yum install firefox2to get Firefox 2.0 installed. The package installs in parallel to the Firefox 1.5 package from Fedora Core, so all those apps from Core and Extras that were build against 1.5 should still work. Note that some of your plugins might not work with Firefox 2.0 and that it will use the same profile directory as Firefox 1.5 (that's different from Gawain's packages -- he started Firefox 2.0 with the profile manager b default, *if* you start it from the menu).
To start Firefox 2.0 simply use the launcher from the menu or start it with "firefox2" from the command line. The package sets a alias "firefox -> firefox2" via a profile.d file, so after the next log in you should be able to just start if by using "firefox".
Special feature: I put a special package into the repo that should obsolete and thus delete the firefox2 package when you update to the development tree or to Fedora 7 later when it comes out -- just make sure the repo stays enabled.
There is one bug ^w feature I don't know how to workaround -- Firefox 1.5 starts up if you click on a URL in a Gnome app in case Firefox 2.0 is not yet running and Firefox 1.5 is still configured as your default browser. That happen to me multiple times, but it did not do any harm to my profile that I got aware of -- but it could, so you have been warned!