Unlike Chrome, Firefox depends on some components of Windows (Media Foundation) to decode MP3 and MP4 media. Apparently some versions of Windows 8.1 do not include all of the media features. I have no idea whehter your is one of those, but Microsoft has a download page where you can get those feature if that is the issue:

• Media Feature Pack for N and KN versions of Windows 8.1 http://www.microsoft.com/en-us/download/details.aspx?id=40744

Edit: According to a blog post I read, Microsoft was forced to unbundle these features and provide the separate download due to antitrust concerns in Europe, so this may be a more common problem than I had realized.

That actually solved the problem! Thank you so much!

I had no clue that MS was forced to unbundled those features. If that's the case then this might be a big problem for Firefox all over Europe.

I mean my friend and i have lurked around on the Internet trying to find solutions to the problem but couldn't find any. And we are actually pretty technical and still we didn't know all this.

Once again thank you!

Thank you for reporting back. I just learned about it this afternoon. We should add it to the help article here if it isn't already in there.

Note that there is a newer version.

Microsoft Knowledge Base (KB) article 2835517 redirects you to this article. This article describes Media Feature Pack update 2929699, which supersedes the previously released update 2835517. In addition to update 2835517, this update also applies to Windows 8.1 Pro N/K for EDU and Windows 8.1 N/K with Bing.

• http://support.microsoft.com/kb/2929699
• http://www.microsoft.com/en-us/download/details.aspx?id=42503