i got it with somebody's help
You can clear cache (through tools>clear private data) before playing song or video then you get less results to analyze.
(NOTE:Be careful.Select only cache otherwise you may lose your saved passwords & browsing history)
The long way:
I found the song of 4.5 MB named as A1E89355d01 in .mozilla/firefox/m0p26gqk.default/Cache/ folder.
i will suggest
$find ~/.mozilla -type f -size +1M
(1)You should have some idea of file size to further reduce results.
) on results to determine file type.
Another better option is run in address bar
Nothing weird just checking in disk cache device search 'mp3' & click on that key then see "file on disk" entry.
Also you can see the mp3 location if you want to wget though no reason to do it because already on disk so why bother.That's it.
I also found out flv file.But here it had 'youtube or whatever your website name is' in keyname.
then rest is same as (6).
I just wanted to explore.It is fun.