Video lag and autoplay

Firefox is up to date. Even when I disable plug ins, I still get lag in the video but the audio streams just fine. What gives?

Also the autoplay is driving me nuts. If I don't remember to pause the video just before it ends so I can comment on it, the next video automatically starts playing and it takes me to a new comment section.

Yes, please turn off the autoplay, or give us a way to turn it off.

The auto-play next video feature should have a box at the top of the video that appears 10 seconds before the end of it - there's a X on this box that prevents it moving to the next video automatically while continuing to play the video to the end. In addition to that, I believe it should be delayed if the video is completely out of view. It will however likely still trigger if/as soon as you can see part of the video but not all of it, and due to the box being at the top it's possible to miss it as a result.