Why do I hear an noise when I alt+tab away from Firefox?

Question

I'm using a few 'normal' Firefox Addins but I am using Vimperator which to me seems like the most likely suspect.

Answer 1

If it is the Vimperator beep, you can try disabling it by putting the following file in your .vimperatorrc file:

javascript liberator.beep = function () { return false; }

Answer 2

Disable the plugin, restart firefox, and see if it still happens.