Firefox scraped the status bar with version 4, the features were basically broken up and relocated elsewhere but surprisingly add-ons got an entire bar to themselves, they get to sit at the bottom of the window where in a horizontal bar spanning the entire length of it. As opposed to that, Chrome’s kept extension icons next to the URL which means they only take up as much space as they have to. url-addon-bar is a Firefox extension that moves the Add-on bar inside the URL bar, icons and all.
What the extension doesn’t do is move the icons that you drag out next to the URL bar from the tool bar customize menu to the URL bar. Only the icons in the add-on bar make it to the URL bar and remain fully functional. Not only does the extension save you a few pixels of space but some might even find it easier to use extension from the URL bar.