Gmail in Firefox is my primary work and personal email client, so I try to make as much space available as possible for Gmail.
I use Hide Menubar add-on which hides the main menubar, then I hide all other extra toolbars and keeping only Address bar (which minimum you need anyways).
Then, recently I found Gmail Compactor Add-On which make further space available for Gmail and make it look little better than standard interface. Make sure you have Greasemonkey add-on installed before you try.
