I observed it before FF 57 was released, on different Windows computers but especially catastrophic on the less powerful ones ( https://news.ycombinator.com/item?id=15688195 or https://news.ycombinator.com/item?id=15687830, I've mentioned that I also discovered that there are complaints at least since February this year, https://www.reddit.com/r/firefox/comments/5sikxt/firefox_unb... only to be downvoted heavily, however let's ignore that at the moment) but now on FF 57 it is provably the same. Just produce a page with more videos autostarting and marvel how much (unbelievably lot!) CPU Firefox uses.

8 videos playing on the page are enough to demonstrate it. Fascinatingly, even if they play somewhere on the page outside of the part which is currently displayed, and even if they are not producing any sound (marked as such) they are going to eat immense amount of CPU.