hi, we are currently aware of this issue on some windows 8 configurations with amd graphics cards and older drivers. please see if there's a more recent graphics driver available for your system at https://support.amd.com/en-us/download and if installing that already solves the playback issue.

otherwise you can do the following to work around the problem: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named media.windows-media-foundation.use-nv12-format. double-click it and change its value to false.

thanks for the quick reply. I used the second option and everything is back to normal now.

The preference name is:

media.wmf.use-nv12-format

in my configuration.