console.log('Application is starting.');
The console.log() method logs a message to the console without providing a line number. Simple but useful.
console.debug('Gets to this point without error.');
The console.debug() method logs a message to Firebug console just like log(), but debug() provides a line number reference.
console.info('DOM is ready, now executing Moo event.');
The console.info() method logs a message to the Firebug console with a blue "information" icon. Line number reference is included.
console.warn('Form field [title] has no value.');
console.warn() sets a warning within the console. The warning provides a yellow background color to easily spot warnings within the console.
console.error('The [title] element is undefined. Bad news.');
The console.error() method places a custom error message within the console with a pink background. Easy to spot.
Each of the above methods accept an "object", or array of objects, and works much like a print-f.
console.log("The %d item has a value of: %d", fifth, myvalue);
This probably looks familiar to a PHP programmer.
Make the most of your Firebug -- send your own message to the console!