UPDATE: Click here for Stable version of Firefox 4.
Firefox 4 beta 2 is already available for Linux, Windows and Mac. You can also install daily builds to get the latest of Firefox. The PPA ubuntu-mozilla-daily provides the daily builds of Firefox, currently beta 3 is available for download. To install the PPA in Lucid or Maverick, just launch Synaptic (System>Administration>Synaptic Package Manager) and then go to Settings>Repositories>Other Software. Now click Add and paste in the following:
Close the Repositories window and then click reload. When it is done, search for firefox-4.0-gnome-support and mark it for installation. It should mark other required packages. Apply and wait for installation to finish.
To launch it, goto Applications>Internet>Mozilla Developer Preview Web Browser or Minefield 4.0 Web Browser and enjoy.