I suppose that you downloaded the tar.gz file from Mozilla's .com or ftp site. I don't think that it is packaged with an installer. There are several ways to run FF4RC depending on what you want if for. Several of these options can be found in this thread: http://ubuntuforums.org/showthread.php?t=330386 If you want daily builds you should add the repository to synaptic (although I heard that this can be unstable). You can find discussion in Ubuntu Documentation here: https://help.ubuntu.com/community/FirefoxNewVersion I don't really want to fool around with daily builds, and am OK running 3.6 as my default browser until FF4 comes out with its final release. For now, I just extracted my tar.gz file with FileRoller and created a launcher for 4RC on my desktop. This launcher is linked with the ''firefox'' file found within the extracted folder. This isn't much of an answer, but I hope that these links help you get this great browser working on your machines.