Firefox provides some kind of adaption to the GTK3 theme.
Sadly this is sometimes limited and sometimes it takes the css part from the “wrong” place in the code.
Here are some examples:
Firefox can’t handle round border edges in its context-menu
And to cut the round corners and keep them for GTK3, we would have to make some chaos in our code, to divide the Firefox context menu from the gtk context menu.
Firefox provides a CSD with its latest stable release (not default but easily accessible and long awaited by many Linux users) - sadly it can’t handle the round edges of GTK3 themes here neither.
A way of changing the theme of the whole window is als provided but in opposite to chrome or gtk3 apps Firefox also themes its headerbar with the applied theme, thus our window-decorations are laid upon their background:
the colors of selected text are taken up from GtkTextView but from GtkView and GtkView is taken for other views as for example the nautilus main view - so we can’t have individual coloring of the text and the icon selection if we want to keep Firefox color consistent to the rest of the theme
the buttons in confirmation dialogues look different to ours (they actually look like gtk2?):
I opened a bug about this here: https://bugzilla.mozilla.org/show_bug.cgi?id=1461621