FF 4 is in beta but it is available for installation and it is much faster.
You get it in addition to FF 3.x with its own settings and add-ons. It is
called Minefield in the menu.

....softpedia.com/.../How-to-Install-Firefox-4-in-Ubuntu-10-10-10-04-9-\
10-and-9-04-183595.shtml