How to install Firefox 3 RC1 in Ubuntu Hardy
To install Firefox 3 RC1 on Ubuntu Hardy, go to System > Administration > Software Sources. Then go to Third-Party Software, click Add, and add this:
deb http://ppa.launchpad.net/fta/ubuntu hardy main
Then open a terminal and write: sudo apt-get update
You should then see the update icon showing you there are new updates available to install… click on it and update your Firefox.
The article where i found this is here.
To find out how to install extensions not supported by Firefox 3 RC1, see the second part of this article.