As u know that Firefox 4 has also been launched officially, so I would recommend you to download the Firefox 4 for Ubuntu / Linux from the Mozilla’s official website first at:
http://www.mozilla.com/en-US/firefox/

and then follow the steps of Installing Firefox below.

Beta Firefox 4 Ubuntu, Beta Firefox 4 Linux

Beta Firefox 4 Ubuntu or Beta Firefox 4 Linux could be downloaded and installed easily by following the steps below:

Download Beta Firefox 4 Ubuntu, Download Beta Firefox 4 Linux

To download Beta Firefox 4 for your Ubuntu or Linux, you can simply goto the Mozilla Firefox 4 Beta url below, which will automatically detect your operating system, and will give you a download link for the tar.gz file:
http://www.mozilla.com/en-US/firefox/beta/

or if you are unable to get your desired version of Firefox Beta, then check the following link:
http://www.mozilla.com/en-US/firefox/all-beta.html

Install Beta Firefox 4 Ubuntu, Install Beta Firefox 4 Linux

After successfully downloading the Beta Firefox 4 file, simply extract the tar.gz file anywhere in your hard drive, for example, you can extract the files into a folder named ff4beta, in your Desktop or home folder.

After extracting the tar.gz file into your ff4beta folder in your Desktop, you simply have to create a Launcher in your Desktop, and start using Beta Firefox 4 for your Ubuntu.

To know how to use Launcher in Ubuntu, check out the following post:
Ubuntu Shortcuts, How To Use Launcher

Just follow the commands as given in the Launcher post given above, and in the Command field, give the path of the firefox file, existing in the folder you just extracted. For example, if you have extracted all the files in the Desktop folder, as ff4beta, then the command path in the Launcher could be:
/home/username/Desktop/ff4beta/firefox
(replace the username with our ubuntu username)

or if you have saved the folder in home folder as ff4beta, then the command path in the Launcher could be:
/home/ff4beta/firefox

Now that you have created the Launcher of your Firefox 4 Beta successfully, simply close your current Firefox, and start using the Firefox 4 Beta by double clicking on the shortcut or launcher you just created.

Hope this will help you downloading and installing Beta Firefox 4 in your Ubuntu / Linux.