Thank you, this report is what got it fixed, and your number was exactly right. On Firefox the live-stream delay captured every frame through a path that copies the picture back to the CPU, and on a 1080p60 Twitch stream that takes longer than a frame lasts, so only 10-13 distinct frames per second reached the screen. Version 1.4.2 keeps the frames on the GPU instead: on the same kind of stream it shows 53-60 frames per second in my testing, with the page back at a full 60 Hz. It was submitted to Mozilla today and goes live after their review. If it's still choppy for you after the update, I'd really like to hear about it.