Imagine your friend that does not know anything about linux, don't you think this would make them not install the firefox flatpak and potentially think that linux is unsafe?
I ask this because I believe we must be careful and make small changes to welcome new users in the future, we have to make them as much comfortable as possible when experimenting with a new O.S
I believe this warning could have a less alarming design, saying something like "This app can use elevated permissions. What does this mean?" with the "What does this mean?" text as a clickable URL that shows the user that this may cause security risks. I mean, is kind of a contradiction to have "verified" on the app and a red warning saying "Potentially unsafe", the user will think "well, should I trust this or not??"
How much sandboxing is your distro generally doing?
beyond root processes, none that I am aware of. Hence I configured all my internet applications and steam to run in a jail :) firejail & bubblewrap come as native packages, unlike the flatpak contents