Am not very experienced but checked add-ons and extensions, but then didn't understand enough of what I read to know if I was on the right track. I can play most videos but am not able to play all videos without getting the error "Playback isn't supported on this device." It even occurs on some "mozilla support" videos in these help topics. This didn't always happen but I can't tell exactly when it began. Most times I'm searching for something specific or short of time and just move on. Can't provide much more except that I'm using Firefox 47 on Windows 7 pro, svc pack 1, 64 bit operation system.