Is there an add-on to capture mp3 files while it is playing on a web site?
There are certain web sites which allow you to play songs but do not have a download option. Is it possible to capture the mp3 file while the song is being played out?
Wšykne wótegrona (1)
Firefox doesn't support mp3, so you're listening to an mp3 only through a plugin. UnPlug, in my experience, usually does the job. But you're free to search for another add-on at the gallery.
If the sound file is vorbis, and is played using open web technologies, then you should simply right-click the player and select the appropriate option ("save file" or something). If it's on the background, I don't know how to easily download it to your computer, but if you look at the page's source code (View > Page source), you should be able to find the direct path to the file. It's just a matter of saving it as you would a normal file.