This seems to be an issue with the Qt framework, as it stands now Firefox is built for Gtk + integration, not Qt -- I think this might depend on your current KDE theme though.

The official builds of Firefox for Linux since version 46.0 from www.mozilla.org/firefox/all/ have required GTK 3.4 (three.four) at minimum to run and therefore needs a GTK 3 theme to theme Firefox.

This sounds like you either have a GTK 2 theme in use on OS or have a GTK3 theme that is buggy which is not uncommon as some theme makers reuse a old theme (that has bugs) as a base for a new theme.

I'm using Breeze Dark for Look and Feel as well Desktop Theme.

Numix-Manjaro for both GTK 2 AND GTK 3 Themes.

Any idea if they are compatible? If not, can anyone make a suggestion. I'm new to KDE so sorry if I sound a little uninformed.

Thanks