Created attachment 631724 [details] Screenshot I've a pdf opened in a window, and the notice is showed on another tab in another window.
See bug 771161 comment 0 for a possible reason why this might be happening.
This happens also on session restore.
pdf.js patch https://github.com/mozilla/pdf.js/pull/1972
Resolved by merged patch above
Verified fixed using Windows 7 and the latest Nightly.