Comment 221 for bug 263435

Honoring the return values of certain JS functions (already) have some ill side-effects. Trap pages (there are a lot of rick-roll-in-the-face domains) exploit onBeforeUnload, onClose event handlers. And they are a massive annoyance, don't make it worse by letting plugins do the same.

A GUI is not necessary, but a few about:config "rules" would allow everyone to control set the behavior to his/her liking.