If Firefox is the default web browser in Ubuntu, something should be done with the thick top panel, either to integrate it with Gnome-shell’s top panel as in Unity or get rid of it as done for Firefox in Windows.
Firefox in Unity
Firefox in Windows
And, Firefox in default Ubuntu
Firefox’s thick top panel might not be a problem, if you are using massive monitors, but with 768px (height) laptops, that’s wasted space, just carrying “Mozilla Firefox.”