Videos in Twitter, Youtube and Facebook (and I'm guessing any video in any website) will not play on Firefox 57 (they worked fine in Firefox 56). They go in buffering mode, the browser freezes for 2-3 seconds, and show a "couldn't play media" error. I tried disabling all add-ons, and disabling hardware acceleration, I even went on and "refresh"ed Firefox to get a fresh profile with no themes or add-ons at all; they still do the same thing. Oddly, they work fine when I restart Firefox 57 in safe mode. Does safe mode disable something other than add-ons? What might it be? Does anyone else have this problem? Edit: The problem is solved. It was hardware acceleration. I simply had no idea I had to *restart* to apply it (because there is no such indication next to the checkbox), and that safe mode disables it (because the hardware acceleration checkbox is actually *checked* in Safe Mode).