On an old machine with few resources, you really should consider fluxbuntu. This “ubuntu”-distro is really fast with the fluxbox windowmanager.
If the X does not fit your needs, you should try Xfce if Gnome (or KDE) runs slowly on your computer. It has a whole lot of nice feature without using up the computer’s resources.
Turn off every handling of IPv6 in Ubuntu for faster internet.
Simply add the file bad_list in /etc/modprobe.d/ with the line:
alias net-pf-10 off
This annoyed me simply too much. It actually made me startup a wordpress blog where I can post my own personal tips – mostly for myself, just to remember!
Everytime I started writing an url, firefox came up with several suggestion based upon history and bookmarks. This got to stop!
Goto: about:config, find browser.urlbar.maxRichResults and set value to 0.
Nomore autosuggestions in the urlbar.