You can type code directly into the Firebug console, and when you press Enter, the code is evaluated and executed. The return value of the code is printed in the console. The code is executed in the context of the currently-loaded page, so for example if you type
document.location.href it will return the URL of the current page.
The console also has an auto-complete feature. It works similarly to the normal command line prompt in your operating system. If, for example, you type docu and hit the Tab key, docu will be auto-completed to document. Then if you type . (the dot operator), you can press Tab several times and it will iterate through all the available properties and methods you can call on the
By using the UP and DOWN arrow keys, you can go through the list of already-executed commands and bring them back in the console.
Type about:config in Firefox's address bar.
Search for strict by typing it in the Filter field and pressing Enter.