Are you playing MP3 files in an embedded QuickTime player or in an external player?

Files played with an external player are always downloaded completely before being past to that player.
With a plugin that plays them embedded it will depend on the player settings for that plugin and with what MIME type the file is send.

Cor-el,

Thanks for your reply, it gave me confidence to keep digging. After checking that the Windows Media Player was installed, working and available for other file types I discovered what I believe is the problem. See “File types it will play” on

https://support.mozilla.com/en-US/kb/Using+the+Windows+Media+Player+plugin+with+Firefox?s=configure+plugin&as=s

I've not tested it but I'm sure this is the problem.

gary