Hello everyone, i just wanted to post this here so others may find it via google. I had a problem with videos randomly playing with black borders. That was really annoying because sometimes it cut out portions of the video. But after months of trying and searching nothing came out, except the "fullscreen black border 'api-config' fix" wich didn't help me. And now finally I figured it out! It was my damn AMD driver...! It was set to "Home Video" in "Audio and Video", wich enabled "AMD video stabilization". So in quick summary: When you have black borders while watching video in Firefox; make sure in your AMD-driver, in "Audio and Video", "AMD video stabilization" is turned '''off!'''. Hope this helps someone, bye :-).