@WPalant @Shamar @zeta @andreas Ditto to Wladimir's points. What you're seeing as a vulnerability, we see as an inextricable consequence of the Web working as intended and as designed.
If you'd like an additional opinion, you're welcome to file an issue at https://bugzilla.mozilla.org
I'm on holiday for the next two weeks, so I'm going to disengage at this point. It good that you're concerned, but your aim is misplaced.