One of the things that Gnome 3 (Adwaita theme) users have been complaining about is Firefox not blending in with other apps, but with the following add-on theme for Firefox installed, it blends in with Gnome 3 apps quite well:

https://addons.mozilla.org/en-US/firefox/addon/adwaita/

It may be worth checking out if you run Gnome 3, Adwaita, and Firefox.