If you are using the Adwaita theme in your Gnome 3 desktop environment – either I have grown to like it or it has been tweaked into something I like lately – the “Gnome 3” Firefox add-on will make Firefox look at home on your Gnome 3 system.
The default icons in Firefox on Ubuntu Gnome are kind of weird: light gray arrows with subtle outline or dropshadow. And these light gray icons are sitting on a bunch of light gray chrome. They are visible, but they are not ideal.
The following Add-on/Theme gives Firefox an appearance consistent with the Adwaita theme, including nice dark-silhouette icons.