I have a small window quickly popping up and disappearing in the top left corner of my screen?

I apologize in advance if this has been covered elsewhere, but I have been searching and haven't found anything yet. And thank you for any help you can offer, it is greatly appreciated.
Every 15 seconds or so a small, empty window will pop up in the top left corner of my screen and then vanish instantly. It opens and closes in a blink, so quickly that I am not able to interact with the window in any way. However, it shows up momentarily as a firefox tab on my bottom panel, so I know it is a firefox problem.
It is only an issue because whenever it pops up I am exited from fullscreen if I am watching a video online, and anything that I am typing while the window pops up will be ignored.
I am currently running Linux Mint Olivia 15 Cinnamon. However, I had the exact same problem when I was running Windows 8. In both cases, firefox has been installed, uninstalled, and reinstalled, which does not fix anything. I have updated firefox since it started happening, which also did not fix anything.
It was very tricky, but I did manage to get a screenshot of the box while it was open.
The same question was asked here, but never resolved: https://support.mozilla.org/en-US/questions/953373

Does this only happen on specific pages?
Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).
*Do NOT click the Reset button on the Safe Mode start window.
*https://support.mozilla.org/kb/Safe+Mode
*https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
You can try to disable notifications as a test (you may want to reverse this).
You can try to set the dom.webnotifications.enabled pref to false on the <b>about:config</b> page.
*http://kb.mozillazine.org/about:config

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