If you log the contents of an object with firebug and then modify the object later in your script the firebug output does not change. But do the same thing with Firefox and the contents of the object will change.

To preserve the object as it was when debugged you have to either copy it or log all the specific properties you're interested in rather than the object as a whole.

Firebug could do it both ways, that was the beauty of console.dir().

Some would probably find that superior to firebug

It's rare to need to see the current (static) state of an object, more often you want to see how the object changes as the code runs.