Some videos won't play, some will. The failures can occur after just a few frames, or maybe a minute or so into the video. It may freeze the video, or the picture may disappear and show a "snowy" screen with the message "Undefined - Info: this browser does not allow you to watch this video. Try to update your browser or use a different browser." One that gives this message and won't play is http://www.cnn.com/2016/12/06/asia/japan-military-pearl-harbor-anniversary/index.html . One that gives this message and will play is https://www.facebook.com/FosterForLouisiana/videos/vb.653107798075203/1325674384151871/?type=2&theater . I can't locate one that stops a minute or two in at the moment. I always get the message "libavcodec may be vulnerable or is not supported and should be updated to play video" but the video may or may not play with the message showing. How do I update libavcodec? This is on Firefox 50.0.2 on Ubuntu 14.04. I think this began after the latest Firefox update. How do I fix this?