This short instruction shows how to install libffmpeg.so in vivaldi to play e.g. mp4 videos.
I use the ubuntu repository for that but this is just one way to get the file.
Go to http://security.ubuntu.com/ubuntu/pool/universe/c/chromium-browser/ and choose the latest chromium-codecs-ffmpeg-extra and download it.
For example: chromium-codecs-ffmpeg-extra_58.0.3029.110-0ubuntu1.1354_amd64.deb