If the files are really that important shouldn't they have good permissions set? That is, if "Firefox shouldn't even have access to" them, shouldn't that be enf

If the files are really that important shouldn't they have good permissions set? That is, if "Firefox shouldn't even have access to" them, shouldn't that be enforced by some OS level permissions? And shouldn't any other programs one has around not be writing confidential information to temp files in the first place? Seems like more of a dumb bug to me than a security problem.