Opps, they still haven't fixed this? This is an unrelated Unity bug and a work around would be to log into classic desktop or gnome-shell do it there. You can add an "affect me too" to the bug report but it doesn't look like they are doing anything about it.
https://bugs.launchpad.net/ubuntu/+s...in/+bug/865672
Ok, something weird that I just found out: the problem does not exist when I am in "Private Browsing".
The exact same video will play on private browsing, and not when I use the browser regularly.
Private browsing creates a temporary blank firefox profile, so... maybe try clearing your browsing history (ctrl+shift+delete). There are also add-ons to clear your flash cache, try that too.
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