firefox playing video at low framerate after update

Firefox just automatically updated and now when playing video while not being the selected window (eg watching a video in the background while doing something else) it plays at a noticeably lower frame rate. I was using my computer today and everything was working fine, when I closed and reopened Firefox it automatically updated, I have changed no settings anywhere on my computer or in Firefox but when i alt-tabbed to a different window while watching a movie after the update Firefox started playing at a low frame rate. Any idea what could be happening? it seems to be caused by this latest update. (I have seen posts from some time ago that seemed to be having a similar problem but the solutions from them do not seem to work).