Firefox as of 46.0 and newer requires GTK 3.4 or preferably newer to run. As a result GTK. 2 Themes will not work as you need GTK 3 themes to theme Firefox. If you change GTK Themes you may need to restart Firefox for theme to fully work in Firefox.

With GTK 2 themes Firefox can look unthemed and the scrollbar arrows be missing.

I have gtk3 installed, it's a fresh testing debian system, and it's the default firefox theme.....

You would not be able to run Firefox 46.0 with out GTK 3.4 or newer.

From your screen shot it looks like it has a unthemed look possibly due to a GTK2 theme in use instead of a GTK3 theme.

Not talking about Firefox themes but GTK 3 themes like these https://www.gnome-look.org/browse/cat/135/ord/latest/

oh lovely gnome-look decided to change their website to be less useful especially in searching.

I just installed some themes from the link above ( Flat-plat, flat-plat-blue, gunmetal etc.) but nothing changed; I can see the bar with the theme adwaita-tiny, but all menus letters becomes white, unreadable.