Firebug is an absolute must for javascript programmers who use firefox. Even if you don't understand any of its complicated features, the firebug console gives

Firebug is an absolute must for javascript programmers who use firefox. Even if you don't understand any of its complicated features, the firebug console gives you good error messages, shows you a link to the line in your code where the error occurred, and you can type in expressions to evaluate them. You want to know what foo.opacity = 0.5 does? Just type it in.