Since the last update of firefox, the kde intégration has changed !It looks like a gtk theme was not applied.
Firefox now uses the gtk3 theme engine. It might be that you have a strange one set or that Firefox in general does only support full theming on newer engine versions.
That’s a minor issue. Until now I used Qtcurve for gtk2 and standard for gtk3.
The look of standard has moved, and I turned to gtk3oxygen engine in place.
I suppose that it is not the latest version of Firefox which use gtk3 theme but that was already the case before.
Thanks a lot.
No Firefox as far as I am aware switched with version 46 to using gtk3 and used gtk2 before
so its the explanation of the change in the look.