I found this write up for the serious bug that was introduced recently in Adobe Flash Player which is splitting and doubling videos here.
Other useful info I found on Adobe's forum last week was that it's also a problem for users with NVIDIA graphics cards -- in that case it isn't just restricted to Firefox but also exists in Internet Explorer, etc., system-wide in all browsers.
Note:
In cleaning tools like CCleaner you'll need to exclude the Adobe Flash Player's settings.sol file so you don't have to constantly re-configure it for the video fix, it's located at:
%userprofile%\Application Data\Macromedia\Flash Player\macromedia.com\support\flashplayer\sys\settings.sol