The GTK file picker Firefox wants to use by default is meh. I'm not sure why it doesn't use the one provided by the OS when it's just one about:config tweak away. Change widget.use-xdg-desktop-portal.file-picker from 2 to 1 and you get the much, much better KDE file picker. Assuming you're using
@kde of course.
It's been a problem for ages on multiple operating systems, not a new thing. At least now you don't have to set environment variables and change startup scripts.