Yes that’s the case. Yours is using the GTK file picker and not the portal file picker.
There are multiple ways to get the KDE native file picker, global env, user profile env, or with firefox about:config > widget.use-xdg-desktop-portal.file-picker=1 should also invoke the KDE native one.
@koshikas Have you tried in terminal GTK_USE_PORTAL=1 firefox and where do you have set your env variable?
The theming issue with xdg-desktop-portal-kde is under investigation upstream. With Wayland it behaves differently.