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??"
I would not be so sure. Firefox has a "save web page as..." option which saves the html page and all other files needed into a sub folder.
Without better handling of reading and writing files the sandbox will break that builtin function. another way of working around this. would be to change firefox to save the web page into one file. Maybe something a .html+zip file that firefox would know how to open. However that would lock other browsers out without manualy unziping it first.
Getting sandboxing right with powerful programs is very hard and I feel the tooling is still not here yet.