I know that you can search for mp3's using this little trick: http://www.tech-recipes.com/rx/851/google_find_free_mp3s_wmas_oggs_and_other_music_files/

But is there any add-on for firefox that allows me to search for files from my search engines bar?

google custom search. I don't know how to make an addon, but it's nothing a bookmark couldn't do. Personally, I don't use the search bar. The navigation bar is enough for me, since it performs the exact same way, except you can't easily change the site to search.