other than debugging your own scripts, you can use it spot some rogue websites.

To download any embeded media files:
1. activate Firebug
2. click on 'net' tab
3. type the file name or file extension in the search box, for Youtube the filename is always 'get_video'.
(or, you can click on 'all' then try to find a file with a large file size.)
4. right click on the file, select 'Open in New Tab'. Most likely it will bring up a 'Save As' dialog, if not, just clke on 'File'->'Save Page As'. You're all set!