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??"
In defense of this warning, when I first put my application on Flathub, I had it because of how file i/o worked (didn't support XDG portals, so needed home folder access to save properly). It did actually motivate me to get things working with portals to not request the extra permissions and get the green "safe" marker.
A lot of apps will always be "unsafe" because they do things that requires hardware access, though, so I could see them wanting something more nuanced.