As with Firefox 50.1.0 the Arc-Maia themes provided by Manjaro aren't compatible with Firefox.
A rebuild/update is necessary.
Hey there - as per the arch wiki I used this addon:
This allowed me to select arc dark
Hi .. sorry for delay i have update the theme ( this is ready for next firefox and i build few day ago ) but unfortunally firefox 51 is only in unstable .. if you want downgrade the theme temporary or update firefox from unstable..