How do I use the developer console?

Enable the console in Options > Miscellaneous > Dev > Enable Developer Console.  You can toggle the console's visibility with ~ (provided that you're not inputting text somewhere).  Type Console.help into the console for more info.

Note that the console has been disabled (commented out) of versions 2.0.5 and greater so it can pass AMO review.  You'll need to uncomment the file if you want to use it.  You can also try the preceding beta version - versions on the beta channel have the console enabled.