If you really want to dig into Firefox customizations and make some interface changes which may not be available through existing themes or add-ons then utilizing the userChrome.css file is what you need to do. Don’t worry, you don’t have to learn a scripting language (unless you want to of course) as there a numerous customizations available for you already.

This is a good tidbit to know because you can apply small changes without having to go out and find and apply a new theme. For example, you can move the URL location bar under the selected tab (just like how Google Chrome handles it).

If you are curious about other uses, just google userChrome.css and see what is out there.