I'm not sure if this is a KDE problem or a Mozilla problem, but I'll ask here in case anyone has had a similar experience.

I've set my KDE workspace theme to McMojave. For Gnome/GTK Application Style I've set Mojave-dark for both GTK2 and GTK3. I check the Prefer Dark Theme button. GTK applications seem to respond as expected (how do you tell which apps are using GTK? GTK2? GTK#?), except for Mozilla Thunderbird and Firefox. There, no matter what application specific theme I apply, I get gray menus with gray text which is impossible to read unless you highlight each menu entry.

Where do I go to figure out how to get Mozilla to respect KDE theme settings? TIA.

Yeah, I'm not sure what the deal is there either. I've also been using dark themes and FireFox; I ended up just unchecking Prefer Dark Theme, and that seems to resolve it for me.