Hi, On testing, libavcodec58 arrived and libavcodec57 has been removed.
Unfortunately Firefox 52 is not compatible with libavcodec58 and needs libavcodec57 to decode H264. Thus, you cannot play h264 anymore. The solution is to manually install libavcodec57 from Debian stable or to upgrade to Firefox 60 which can use libavcodec58. Unfortunately Firefox 60 is not yet on testing. Maybe Mike Hommey can give further info on why the migration is blocked. Best regards, Bugs #909722 #909398 and #906567 are duplicates of this same issue.
