Created attachment 8450209 [details] The menu button is pressed (it's on the right) and the menu opens on the left monitor. (The background image indicates the start of the second monitor.) Steps to reproduce: 1) Find a computer with two monitors and open Firefox. 2) Resize the Firefox window so it does *not* fill the full screen and you can move it. 3) Move Firefox to the right end of the right monitor so that only 1/3 or less of the menu button (≡) is visible. 4) Click on the menu button, so that the menu opens. Expected result: The menu should open underneath and a bit left from the menu button so that the menu is fully visible. Actual result: The menu is shown on the right side of the left monitor. (See attached screen shot.) The position itself is okay, but it's the wrong screen. I've tested it in the current stable version (31.0) and nightly version (33.0a1 (2014-07-03)). It seems like all menu panels (bookmarks, more tools, site info, ...) are affected.
It's still reproducible. Tested with the current Firefox Beta 40.0 and Windows 10. => Changing operating system to Windows in general and status to confirmed.
Status: UNCONFIRMED → NEW
Ever confirmed: true
OS: Windows 8.1 → Windows
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