I checked firefox in manjaro KDE 16.06 and it's ok. Firefox in Manjaro 15.12 (46.0.1) and Manjaro 16.06 (45.0.2) have this "bug".
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It is not a "bug." GNOME.org developers, as you are probably aware, recently changed the GTK3 CSS API, making older, non-compatible themes display like that. The simple solution is to change the theme you are using. Again, there is no "bug."
Regards
Regads