I have had several requests in the past from people wanting to know if I knew of an extension that would help you add sound effects to tasks in Firefox.
I never did spend too much time on this request because personally I like my browser to be nice and silent. People with special needs though could use an add-on like this so my search started. Looking high and low, I found one add-on that promises to do the job, the Noise extension for Firefox.
Here is a little more about the Noise add-on from the developer:
After installed and you have restarted Firefox, sounds should be ready to response events. You can try open Findbar (Ctrl + F) and find a word, when the find reaching the end of page (when hit F3 to find nexts), you can heard a “beep” sound.
To edit the noises and events, go to Add-ons Manager (via Tools > Add-ons Manager) and find Noise. Enter the Preference Window via the Option command. More help for using the add-on can be found at the official web site.
Extra Tip: Might also try these BugMeNot usernames and passwords if you don’t have the time to register on Mozilla’s site to get access to sandboxed add-ons.