The other day I was trying to play a track on Soundcloud and it wasn’t playing. Soundcloud switched to an HTML5 player instead of Flash quite a while ago. This was strange because it had previously worked. I tried Safe mode but it still didn’t work. I tried Chrome and it worked in there so I was somewhat perplexed. I eventually found that starting with Firefox 30, Firefox started using Gstreamer 1.0 instead of the old 0.10 version. By default not all of the plugins to Gstreamer are installed in (K)Ubuntu. Pretty much all of the open source sound codecs will work (ogg, flac, vorbis, etc) but mp3 will not. The fix to this is simple.
sudo apt-get install gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly
Enjoy your HTML5 mp3 playback!