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??"
By not letting the user import/export addon settings, bookmarks?
Btw, i hate the opinion that the dev must babysit his users. It makes software worse, not better, look at Firefox's profille folder for an example. If you have to, make an intro to train them.
I'm not 100% confident but I thought you could use portals to access individual files outside of the sandbox
You could but where is fails is when you open one html file that then needs to loads the other files that are needed by the first.
You can not allow chain loading like this, it would bypass the sandbox.
One way of working around this would to allow the option of passing a whole folder and sub folders to the program.
The other and much harder option would be a per program portal filter that can read the html file. then workout what files that html file needs and offer that list of files to the user.
The lazy work around is allow read access to $HOME and deny access to some files and folders like .ssh
You can choose folders in the portal now.