Under Flatpak we cannot determine if the application is set as default handler
for http/https protocols and also the application cannot set the system defaults.
In order to avoid user confusion we hide the defaultBrowserBox which
under flatpak shows always: "Firefox is not your default browser" and the
"Make Default..." button does not make any change to the system.
Differential Revision: https://phabricator.services.mozilla.com/D170590
7b0bbd4dca