Firefox 4.0 Final stable version released today opening a completely new browsing experience for all internet addicts. For Linux users its a new convenience that the browser gets updated automatically without any Terminal updates keeping it safe from bugs and threats.
Here you can install Firefox 4.0 without using a repository, a Terminal command to install this version while keeping the previous version.
chmod +x firefox_4.0_i386
chmod +x firefox_4.0_amd64
Alternatively, you can test / install the new Firefox 4.0 on Ubuntu or any other Linux distributions just downloading the file. tar.bz2 from links below.