What this person told you was wrong, you need to use flatpak run [package id] to run flatpak apps. You can do flatpak list to see all installed flatpaks with their ids. An id looks something like org.example.app and you’d run it with flatpak run org.example.app.

Also, is it shown as being installed in Discover? If it’s not you could try just installing it again and if it is, you can try uninstalling it first. The user data for Firefox should stay intact.

Why? It’s just a relatively new image format and it’s an improvement over what was there before