Bear with me as I've been using Linux for 2 days, but this process helped me understand a lot more about the OS and update process...
But already I want to "give back" so:
Assuming your XBMC session is running well for you...
Exit to a terminal session
you need to get to the terminal prompt with your admin/login/xbmc password
it will read
approve any Y/N confirmations and let it install your gnome desktop
sudo apt-get install gnome-desktop-data
once complete type
there might be some odd error about "wallpaper/eterm" click OK...it ain't pretty but follow me here....
right click on the desktop select xterm under Applications:Terminal Emulators:xterm
that will open another terminal session
The next several commands will throw Y/N confirmations about disk space at you just approve them all type
sudo apt-get update
sudo apt-get install update-manager
sudo apt-get install firefox-3.5
now to launch from that beautiful desktop select Firefox 3.5 Browser buried under Applications:Network:Web Browsing:Firefox
EDIT : see jgs2n's post immediately below to setup a launch/icon within XBMC
Was that as good for you as it was for me?