I don't know what's causing it, but whenever I set a YouTube video quality to 720p or 1080p Firefox slows down drastically and the video will freeze frequently and get quite jerky. When I put the video into fullscreen on 720p or 1080p, it slows down monumentally more... as in it takes a few seconds to pause when I click or hit spacebar & it takes a few seconds for the UI to show up. If I wait for the ENTIRE video to load, it will continue to play fine. I just don't know why it's being slowed down so much. I checked activity monitor and CPU usage is high but it's not capped, it's around 60-80% in these cases. I've already tried Resetting Firefox and updating Flash Player. Memory is typically between 30-50% usage. My computer specs are: Core 2 Quad Q6600 @ 2.4GHz, 4GB DDR2 RAM, GeForce GTX 560 1GB hardware acceleration is on. Any ideas what's slowing it down and what I can do to fix it? Thanks.