Do you generally download videos from many sites? I only download from YouTube myself, and so I use YouTube Video and Audio Downloader, which has a WebEx version compatible with Firefox 57. It needs you to install some extra programs to automatically combine the (otherwise separate) audio and video streams in HQ YouTube videos, though.
Like you, I also disabled all updates the day before FF 57 was released. I'm disgusted by Mozilla releasing such a broken, unfinished version as an automatic update. Even more disgusted that I forgot to turn off the updates for my phone and now can't get rid of the hideously white new style and broken theme support there.
(Luckily, Firefox 58 will at least allow you to get rid of the blindingly bright white top bar)
There's my earlier rant thread in case you missed it.