I've recently noticed that Firefox is sending the following error message to the web console for a web application that I'm developing : Cannot play media. No decoders for requested formats: video/mp4; codecs="avc1.640032 mp4a.40.2", video/mp4; codecs="avc1.640032 mp4a.40.2", video/mp4; codecs="avc1.640032 mp4a.40.2", video/mp4; codecs="avc1.640032 mp4a.40.2" This is happening in Firefox version 84.0 (64-bit) Windows 10. I restored a Windows system image made October 15th, that had Firefox version 81.0.2 (64-bit). The error message does not occur there using the same application software that does receive the error. The error message doesn't seem to cause any issues. The 4 video streams still play normally. Does this error message indicate that there is a latent error in my application code that I need to resolve? Can you give an ideas what might be the cause of the issue? Thanks for any help in advance.