Same here. Previously, when I'd finished watching a video / wanted to stop watching, I could click the "previous window / go back one page" arrow (upper left, under "firefox view", and the video would stop playing and revert to the previous page (subscriptions, home, etc.). Now, when I click the arrow, the browser restarts the video. Clicking it again still doesn't return me to the previous page (restarts video again). Clicking again after twice failing to open the previous page returns me to my home page (skipping the subs or home page I'm trying to get to). I have to launch YouTube again in order to see my subs or home / suggested videos.