1. Find where your Firefox’s cache directory is. You can do this by typing “about:cache” into Firefox’s address bar.

2. Firefox will display a new page called “Information about the Cache Service.” Under the heading “Disk cache device” look for the “Cache Directory.” The directory listing will likely be quite long.

3. Navigate to the directory. You can do this on a PC by highlighting the directory with your mouse cursor then pressing Ctrl+C or right-clicking your mouse to copy the text. Next open the START menu and select RUN. Paste the directory into the dialogue box and click OK. On a Mac, open a Finder window and navigate to the directory by following the path listed.

4. Once you are in the cache directory make sure that the window is set to display file sizes and the modified dates of files (so that you can see the dates/times that files were added to the directory). In Windows you can change to this setting by right-clicking in the window and selecting View then Details. On a Mac you can do this by switching the Finder window to List View mode.

5. Leave the cache directory open. Now navigate your Firefox web browser to a website that streams mp3 music. Start listening to a song and let it load completely. Once the song has finished loading wait a few minutes then go back to your cache directory and refresh it (right-click then select Refresh on a PC or select View then Refresh in the Finder window on a Mac).

6. Look for the most recent files and find one that would be about the size of your song this will usually be in the range of 2MB to 7MB depending on the length of the song. The file name will look like jibberish but this is the file you want.

7. Select the file then drag it onto your desktop. Rename the file and give it the extension “.mp3.” You can do this on a PC by selecting the file, right-clicking on it, and selecting rename, or by selecting the file and pressing the Enter key on a Mac. Once you’re done the file name should be something along the lines of: “Songname.mp3”

8. Now put the file in your .mp3 player software and you’ve got your own copy of the song you were streaming.

Heaps easier,

1. empty cache (preferences/network/clear cache)

2. play file in Firefox and wait till it loads

3. open new tab, in address bar type "about:cache"

4. Open "List Cache Entries"

5. Look for file (should have http address and long file name with a file size around 5-10mb depending how long the song) 

6. Open it/Click on the blue address (should start to play the file)

7. Click the down arrow on the play bar to dropdown list and select save as source

8. rename with extension .mp3