Firefox To Play Ogg

Question

I have an ogg/audio file converted from mp3 using Alt WAV MP3 Ogg Converter. When I drag and drop the file in Firefox or in Google Chrome, the audio plays nicely. But when a use the following code to place the file inside a page it only works on Chrome.

My code is simple as that:

<audio controls>  
     <source src="foo.ogg" type="audio/ogg">  
</audio>

Someone can tell me why?

Thanks