After a website had entered fullscreen mode, it could have used a previously opened popup to obscure the notification that indicates the browser is in fullscreen mode. Combined with spoofing the browser chrome, this could have led to confusing the user about the current origin of the page and credential theft or other attacks. This vulnerability affects Firefox < 74.
74.0+build3-0ubuntu0.16.04.174.0+build3-0ubuntu0.18.04.138.8.0~repack1-0ubuntu138.8.0~repack1-0ubuntu338.8.0~repack1-0ubuntu452.3.1-0ubuntu352.3.1-7fakesync152.8.1-0ubuntu0.18.04.152.9.1-0ubuntu0.18.04.174.0+build3-0ubuntu152.9.1-1build152.9.1-1ubuntu3Exploitability
AV:NAC:LPR:NUI:RScope
S:UImpact
C:NI:LA:NCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N