True, although not in the same way. KDE is using its own GTK theme and uses ~/.config/gtk-3.0/gtk.css to override colors to the accent color. But this method is broken for sandboxed versions of Firefox since they can’t access ~/.config/gtk-3.0/gtk.css (though with flatpak you could create an override to allow it).

I believe (and hope) Firefox is now following the standardized accent color portal for determining the accent color. If so, then this accent color change should work on Gnome, KDE, Pantheon, and other desktops that support the accent color portal. If true, then even sandboxed apps should follow the accent color without messing with the sandbox.