It was still happening for me on Firefox, but I tried disabling my addons and it turned out to be caused by an addon called SmartVideo for Youtube. After disabling that, it's no longer an issue.
Specifically, I get a "This page is asking you to confirm that you want to leave - data you have entered may not be saved." modal dialog in Firefox, preventing the page from closing and preventing me from doing anything else with the browser. I have not entered any data and I want to leave. If I didn't, why would I have closed the page?
I seem to get this in every page with a video in it, even if the video is not running. I also get it if I try to go to another page. It's been going on for maybe a few weeks.
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