It’s true, and here’s how you do it:
First, open up Firefox and type about:config in the address bar and hit Enter. You’ll get a warning screen that looks like this:

Click “I’ll be careful, I promise!”
From the list of preferences that appears, scroll down and find layout.spellcheckdefault and double-click on it.

Change the value to 2 and click OK.

Now restart Firefox and enjoy the functionality of an add-on without actually adding anything! If you ever want to go back to the default setting, just follow these steps and change the value to back to its default setting: 1.
Awesome!
~Andrew

