Jim P.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.