The "Go" button in Firefox is rarely used and takes up space on the screen, although not all that much. If you want, you can easily disable it to give your address bar more room.
Notice the little green useless button on the right side.
Just type in about:config into the address bar, and then filter by the following value:
Just double-click on the value in the list to change it to true. The green button should instantly disappear, as you can see in the screenshot.