I've had this problem for awhile in firefox. I'm using a MacBook Pro (Retina, 15-inch, Mid 2014 16 GB 1600 MHz DDR3) and when I play videos on youtube or elsewhere, the video slows down and lags behind skipping many frames. The only way to resolve this is to restart the computer and then it works fine for awhile but then after awhile continues to do this again. Any ideas how to resolve this?