This is posted in order to provide an answer.
Details about the initial problem are here.
It involved not being able to play Youtube live streams, Tweeter live streams, Vimeo non-live streams and many other online video in neither Chromium or Opera. (Now I have found a fix for Chromium that I am posting here, while I keep the other question for Opera only.)
After installing chromium-codecs-ffmpeg (as indicated in an answer under the linked question) this was not fixed; but installingchromium-codecs-ffmpeg-extra instead (installing one package removes the other) for the purpose of playing Vimeo videos (as indicated here) solved for Chromium all the problems I had with both Opera and Chromium (not playing live streams and a lot of other non-live online video).
(N.B The problems were solved only for Chromium, not for Opera.)