Using FireFox. While on flathub.org if you click ‘install" under an app name, a firefox pop up ask "What should Firefox do with this file’ if you select "Open with’ click other and select the “Software app” when the App opens this error is displayed "Failed to install: not supported’
Doesn’t really work correctly by default in Clear Linux.
I’d use the gnome-software manager or command line install for either system or user for the program you wish to install.
Guess there is a way to download that .ref file and run it from command line too.
“flatpak-flatpakref — Reference to a remote for an application or runtime” is what you are clicking on in firefox.