Firefox 4 introduced many changes to user interface that includes tabs on top, new menu interface, status bar in address bar , new add-on bar etc.
While many UI changes are really nice, the add-on bar at the bottom is a somewhat big and cannot be resized.
This can be fixed using a Firefox add-on Firefox 4 UI Fixer that allows users to move add-on icons to any toolbar they like. All you have to do is drag and drop all add-on icons to some tool bar and then disable the add-on bar.
Download Firefox 4 UI Fixer from here