see these addons to remove gmail addons :
1. https://addons.mozilla.org/en-US/firefox/downloads/file/16877/stylish__45_0.5.2__45_fx__43_tb__43_sm__43_fl.xpi
For opera browser users and who want to use this tweak in firefox without addon go to the following page.
http://techvi.blogspot.com/2007/09/how-to-remove-ads-from-gmail.html