This is the Beginning of a Beautiful Friendship
console.debug("This is a message.");
Any errors that occur, missing variables, missing functions, anything along those lines are logged to the console making troubleshooting extremely painless. What could've taken minutes or hours of dropping painful alerts in the code to see what was going on simply did not occur.
The newest beta of Firebug will also print out any and all HTTP requests happening inside of the browser. These print out in the console, listing all headers sent to the server, received from the server, the body content of the request and response and also the amount of round-trip time for the request.