Audio playback and rendering of all related graphical elements on this homepage require your browser to support .mp3-files.
Microsoft’s Internet Explorer, Google Chrome and Safari should support .mp3-files natively.
Firefox should by default also enable .mp3-playback by using local ressources.
If this is not enabled, type “about:config” in the adress bar of firefox, search for “media.windows-media-foundation.enabled” (on Windows), or “media.gstreamer.enabled” (on Linux) and set it to “true”.
Linux & Firefox-specific
gstreamer should be available for all major Linux distributions and in most cases is already onboard by default; however, in some cases (e.g. on Arch Linux) the critical gstreamer-plugin for .mp3-support, “gst-libav“, needs to be installed separately (e.g. here for Arch). On Ubuntu, this plugin seems to be contained in “ubuntu-restricted-extras”; to install it (on Ubuntu), type “sudo apt-get install ubuntu-restricted-extras” in the Terminal.
If it still doens’t work, check that the basic codec plugins for gstreamer are installed as well (e.g. gst-plugins-good & gstreamer0.10-base & gstreamer0.10-ffmpeg & gstreamer0.10-good & lame; not sure all of them are needed)
In principle, the same should also apply for Firefox on Android.
For further information regarding .mp3 in Firefox also refer to this discussion page.