Do the following:

• Open the Firefox Options
• Select the Applications tab
• Locate all MP3 file types
• Change each MP3 action to Other...
• Select Windows Media Player from the list
• Click Ok
• Restart Firefox

This should open all MP3 files in Windows Media Player.

That's what was selected and I've even changed the option to download and the links still opens an in-browser player. The last time this happened it was because of the "windows-media-foundation player" being enabled which had to be disabled in "about-config". As far as I can tell that option is still disabled so it must be new code added in the last update.

I found, by accident, changing to "false": "media.directshow.enabled" in about:config resolved the problem. I'm not sure why, since I'm on a Windows 8.1 (64) PC running the latest version of WMP, and the article I found this on was related to Windows XP. I have to assume if I had disabled this before then the latest update turned it back on.

For Windows Vista and later this would be the media.windows-media-foundation.enabled pref.

That's how it worked before, but that option had already been changed before (on version 20 something or the other, whenever that got added) and was still disabled when I went to change it (again). As I mentioned above the thing that fixed it was changing "media.directshow.enabled" to false (which according to everything I read should only affect Windows XP PCs, my PC is running Windows 8.1 (64) )