Solution for Manjaro/Arch Linux.
In my case it doesn't work to play videos on facebook. Thank you to the user of the @appdevsw, he led me to the right track. I tested my solution at Manjaro. Probably it will also work on Arch.
Install the missing codecs: opera-ffmpeg-codecs.
https://www.archlinux.org/packages/community/x86_64/opera-ffmpeg-codecs/
After installation, restart the opera.