You can check for issues with "xdg-desktop-portal" and possibly install some packages. sudo apt ins

You can check for issues with "xdg-desktop-portal" and possibly install some packages.

sudo apt install xdg-desktop-portal xdg-desktop-portal-gtk

Whether to use gtk portal for the file picker. 0:never 1:always 2:auto (true for flatpak or GTK_USE_PORTAL=1, false otherwise)

about:config => widget.use-xdg-desktop-portal.file-picker = 0

https://support.mozilla.org/en-US/kb/about-config-editor-firefox

https://support.mozilla.org/en-US/search/?product=firefox&w=2&q=%22xdg-desktop-portal%22