In ubuntu you'll get the latest by default but you'll need a few more steps in order to get RC1. In System>Administration>Software Sources click the 'Third-Party Software' tab and the 'add+' button then type in
deb http://ppa.launchpad.net/fta/ubuntu hardy main
You should be prompted automatically that new updates are available. Click the 'Install Updates' button then the rest is automatic.
Interesting enough if you look in Help>About Mozilla Firefox you'll see it's Version 3.0 without mentioning RC1.