Although you do not reference the site from which the error message arose, I suspect that you need to install a special application, x-mplayer2, to listen to the samples. Look around the site for information about where to download this.
You should have WMP, QT, and Realplayer already installed, as well as Adobe. These are not exactly "extensions" like many of the small ad-ons available to Ff users, but rather stand alone applications that work with any browser.
You will need to download JavaRuntimeEnvironment, since the IE "in-house MS version of Java does not work with Ff.
Here is a link to the list of "plug-ins" needed for Ff, and links to their download:https://addons.mozilla.org/firefox/plugins/