Your friend is maybe using an add-on to play MP3. e.g. MP3 player add-on

No, he uses Firefox just because he can't run Chrome properly and he hasn't changed anything or installed any add-on.

Which web pages with MP3 HTML5 coding don't work for you?

MP3 on this HTML5 Audio Formats Test page works for me (Fx24, Win7)

http://hpr.dogphilosophy.net/test/

If the MP3 test on that page doesn't work for you, enter about:config in the Firefox address bar and make sure that the preference
media.windows-media-foundation.enabled is set to "true", which is the default setting.

Ref: http://kb.mozillazine.org/File_types_and_download_actions#Built-in_support_for_certain_media

No pages which use MP3 HTML5 or playing MP3 directly (which is actually the same) work for me.

I had already tried that page and it says it doesn't support MP3.

media.windows-media-foundation.enabled is set to "true".

Are there any error messages in the Web Console (Firefox/Tools > Web Developer;Ctrl+Shift+K) about these mp3 files (common CSS errors are not important and can be ignored)?

• https://developer.mozilla.org/en/Tools/Web_Console

Does Firefox retrieve the MP3 file?

You can use the Network tab in the Web Console or this extension.

• Live Http Headers: https://addons.mozilla.org/firefox/addon/live-http-headers/

In the page AliceWyman posted, if I click the try button, this appears in the Web Console:

[11:25:45.759] GET http://hpr.dogphilosophy.net/test/mp3.mp3 [HTTP/1.1 206 Partial Content]

[11:25:46.818] HTTP "Content-Type" of "audio/mpeg" is not supported. Load of media resource http://hpr.dogphilosophy.net/test/mp3.mp3 failed.

In the Network tab I get:

• Green circle, if I move the mouse pointer over it it says "206 partial content".
• Method: GET
• File: mp3.mp3
• Domain: hpr.dogphilosophy.net
• Type: mpeg
• Size: 0 KB

I see you posted an earlier question about his problem, on Oct 8, 2013,

• /questions/973656 I can't play MP3 audio directly or with HTML5 on Firefox

You wrote in that thread that direct links to MP3 files such as this mp3 example, also don't work for you in the Firefox built-in player.

You also wrote,

I have a laptop with Windows 7 Home Premium x64, which I had to reinstall recently because of some hard drive problems. I used to be able to play MP3 audio files directly on Firefox, when I clicked a link to a MP3 file a new tab would open and the audio would start playing. But now I get a window asking me whether I want to open it (with Windows Media Player by default), or save it, and I can't play MP3 files with HTML5 <audio> tag, supposedly my browser doesn't support it. 

I'm thinking it might have something to do with the fact that you recently reinstalled Windows 7. Did you also install SP1 and later Windows updates?

Yes Firefox uses your system's codecs to play media files, so your system needs to be somewhat updated.

Yes I asked that question some days ago but I thought I'd better rewrite it to see if I could get any more help.

I think I had SP1 installed, though I can't remember it. I'll try installing it now to see if it works.

@AliceWyman :

I installed Service Pack 1 and now it works like a charm.

Thank you very much!

Hey, that's great! You're welcome!

Thanks for posting back with the outcome.