First, you need to make sure that you have Quicktime installed. It comes bundled with Itunes and can be downloaded here
After the installation is complete, open quicktime and press on Edit->Preferences->Quicktime Preferences. This will allow you to chose the file formats you want quicktime to play while you browse the internet.
Choose those you want quicktime to play, restart FireFox and voila! You should be able to listen to .mp3 files and videos with out a problem.
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