class js.html.Console

Beginning with Firefox 4, the old Error Console has been deprecated in favor of the new, improved Web Console. The Web Console is something of a heads-up display for the web, letting you view error messages and other logged information. In addition, there are methods you can call to output information to the console, making it a useful debugging aid, and you can evaluate JavaScript on the fly.

The Web Console won't replace more advanced debugging tools like Firebug; what it does give you, however, is a way to let remote users of your site or web application gather and report console logs and other information to you. It also provides a lightweight way to debug content if you don't happen to have Firebug installed when something goes wrong.