Most of the components of the Firefox user interface can be removed, added or moved around easily. This is quite a difference from Chrome, where such functionality is quite limited.
For example, I like to hide the search bar, remove or move around the buttons that my extensions put in the toolbar.
Doing all this is easy. Just go to Settings -> Customize. Now you can grab any UI component (bar or button) and move them between the toolbar, the Additional Tools and Features window or the Customize window.
Tried with: Firefox 41 and Ubuntu 15.10