Firebug send NULL value when value is an accent Latin char encoded.
Ex : fb( 'éèàù','accents',FirePHP::INFO);
In FireBug Console -> accents : NULL
I had to patch FirePHP.class.php line 1385 :
return (mb_detect_encoding($str,'UTF-8',true) == 'UTF-8');
So in firebug console -> accents : éèàù