Standard checks: -OS is up to date -Graphics drivers are up to date -Firefox is up to date Attempted fixes: -Disabled hardware acceleration, problem seemed to get worse -Set layers.acceleration.force-enabled to true, no effect at all -Fiddled with resolution of video, no effect at all, even 240p dogs Problem seems to have begun at or shortly after switching from Win7 to Win10, but does not seem related to Win10 settings, as video playback works normally in MS Edge. Other websites with HTML5 video (tried twitch and crunchyroll) do not have the framerate problem, but do seem to have some odd responsiveness issues when going in and out of fullscreen.