Hello,
I must use FireFox Nightly (due to an extension without signature).
If I type "console.log("test");" in FireBug command line, I have only the command line (blue caracter) and the word "undefined".
The result of the command (printing "test") appears in the Firefox console (CTRL + SHIFT + K) and not in the FireBug console.
You can see the screen shot.
Thank you and sorry for my bad english.