Firefox position is completely valid. I think a safe option would be to allow access only to a specific directory like "~/Internet files" or something like this. This way the user could grant the access but not to sensitive files. And add an option in about:config to lift the restriction for power users.

Also, there is a risk of a site writing malware executable, and Linux currently has no sandboxing for such executables so the system would be completely owned once the user runs the program. So the directory should not allow storing executables.