select element persistance allows for attacks — Mozilla

Security researcher David Bloom of Cue discovered that elements are always-on-top chromeless windows and that navigation away from a page with an active menu does not remove this window.When another menu is opened programmatically on a new page, the original menu can be retained and arbitrary HTML content within it rendered, allowing an attacker to cover arbitrary portions of the new page through absolute positioning/scrolling, leading to spoofing attacks. Security researcher Jordi Chancel found a variation that would allow for click-jacking attacks was well.