Pop Out Video → Full Screen → Exit Full Screen (via OS X menu bar) leads to invisible video.

[Wowfunhappy]

To reproduce:

1. Open a Youtube video (or likely almost any video), such as https://www.youtube.com/watch?v=WO2b03Zdu4Q
2. Click the "pop out video" icon.
3. Click the Fullscreen icon in the bottom right corner.
4. Exit full Screen by:
   1. Hovering your mouse at the top of your monitor to unhide the OS X menu bar, then clicking the icon in the top right corner to exit full screen.
   2. Pressing the esc key.
   • Do not click Firefox's "exit full screen" button in the bottom right corner, which won't trigger the bug.

The video will disappear. The tab will say "the video is playing in Picture-in-Picture" mode, and sound will be audible, but there's no window to be seen.

Additionally, if you follow steps 1–3, then switch back to the space which contains your Firefox window while the PiP video is still full screen, the PiP will display over the Firefox window and everything else. I'm not actually sure what the expected behavior is here—I'd hope it's just "let the video keep playing in its own full screen space and don't interfere with other spaces" except Mozilla and I don't always agree—but it clearly shouldn't do this.

You can "fix" these problems by setting full-screen-api.macos-native-full-screen to false in about:config. However, you loose some potentially nice OS X integration in the process.

Tested on 10.9.5. I'm on my main system rather than a VM, but I can test in a clean VM if you really think it's system specific.