Revision to previous comment Rated 5 out of 5 stars
For instance, two popular sites that display 1-2 pixels more height on the address bar are addons.mozilla.org and facebook.com. I should add that I have small icons for the taskbar, other than that there are no settings changed. Regarding addons, which kind could affect taskbar height? I'm not using any tabs addon, merely an addon that restores page title to the titlebar.This review is for a previous version of the add-on (126.96.36.199-signed). This user has other reviews of this add-on.
As you can see on my first screenshot and on my second one, the mentioned glitch doesn't occur on my system. Tested with a clean profile on Win7x64. http://imageshack.us/photo/my-images/842/88540190.png/
Could you disable all your extensions and test GMF again? Please use the new support site for further reports. ;-) http://forums.mozillazine.org/viewtopic.php?f=48&t=2410445