To the best of my knowledge this is only an issue with Peppermint 6 and Firefox 50.
FF 50 users will notice the following warning on many websites: "libavcodec may be vulnerable or is not supported, and should be updated to play video."
What it means: Libavcodec versions older than 54.35.1 will be blocked by Firefox due to security concerns. Ignoring the message will sometimes work, sometimes not.
How to fix it:
Open Terminal and type the following commands:
sudo add-apt-repository ppa:heyarje/libav-11
sudo apt-get update
sudo apt-get install libav-tools
sudo apt-get upgrade
Press 'Y' (Yes) if the installer asks you about replacing old files.
A reboot is recommended afterwards.
