I have noticed that I have not been able to play audio or video from a few different players in Firefox for a while, namely videos from the Canal + Zapping website and music from the Free Music Archive website; however, most other websites, like Youtube, Dailymotion and Soundcloud, work perfectly in Firefox.
Also, this problem does not exist in Google Chrome.
I am using Ubuntu 13.04 and Firefox 22.0 with proposed and backport updates, as well as ubuntu-restricted-extras 57
installed.
Cheers
It was actually the add-on HTTPS Everywhere that was the culprit. I could listen to the Free Music Archive after disabling it.
The Zapping website ended up working fine after a while, without any particular action.
I might add that streams using mp4, like this one, because of licensing issues (they will support it when a third-party decoder is available).