Open up Firefox. It doesn't matter at this point what website (if any) that you go to, since we will be writing a stand alone code.

Open up Firebug by clicking on the Firebug icon. Go to the Console panel. If it is not enabled, enable it.

Now, at the bottom of your screen you'll see an area where you can enter code, designated by >>>. Clicking anywhere after that will allow you to enter the code.

Type in the following code, and then hit Enter.

alert('Narwhals like bacon');

You should see an alert box pop up with the text Narhwals like bacon (or whatever string you passed into the alert function). After the code is executed, it will appear in the log above the input line.