One of the most useful features are the logging functions through console.log which can simply be used to output a string or object:
console.log('Testing log output: ', myObject);
This will output your message and the full object available for inspection in the Firebug console tab. In addition to console.log, there is also console.warn, console.error, and more. See the Firebug documentation for more info.
console.log in action
You can also add a conditional breakpoint by right clicking on a line number. Now the program will only pause under your specified condition.
Setting a breakpoint