You just need to add the following to your /etc/apt/source.list:
deb http://ppa.launchpad.net/fta/ubuntu hardy main
Then you only need to update (with sudo apt-get update) and then
upgrade (with sudo apt-get upgrade) to install the firefox-3.0 RC
When Firefox 3 is in the regular Ubuntu repo’s, you won’t need this anymore…but it won’t hurt anything to stay in sources.list.