When apps are maximized in Gnome 3, the window title bar is supposed to hide/blend into the panel/menubar like in Unity. Lots of apps don't do it yet, though.

For Firefox, try this extension to hide the title bar when maximized: https://addons.mozilla.org/en-us/firefox/addon/htitle/

While you're at it, if you're on Gnome, try the excellent Gnome 3 theme: https://addons.mozilla.org/en-us/firefox/addon/adwaita/