Install Greasemonkey, that will allow you to run custom scripts on top of your web browser.
Then install this custom script that will remove extra white space in your Gmail window:
Simply click Install button at the top right corner of the page.
Restart the Firefox.
EDIT: It works just fine but may get unstable so that it will log you out from Gmail after some time. Still it is worth of a shot if you don't like extra white space.
EDIT 2: Now you can just go to the Settings (click the wheel in the top right) and select Display density: Compact