How to use WP-FirePHP extension?

http://wordpress.org/extend/plugins/wp-firephp/

I’m simply supposed to call fb('Error message','Lable') but it doesn’t work all the time. I can’t figure out when and where the relevant files are being included and classes defined in order to make the call to the said method/function.

For example it doesn’t work even when I call fb() from wp-content\plugins\wp-firephp\FirePHPCore\fb.php itself and open up http://localhost/wordpress (maybe because the file never gets included) but it works on http://localhost/wordpress/wp-admin. Does that mean wp-firephp plugin only works for /wp-admin and related pages? Because I wanted to use its functionality on non-admin related pages like /footer.php and such. Am I doing it wrong?

I gave up on using the plugin and use FirePHP straight as a mu-plugin:

And firebug.php file consists of:

<?php
/*
    Plugin Name: FirePHP
    Version: 0.1
*/

require_once( 'FirePHPCore/FirePHP.class.php' );
ob_start();
$firephp = FirePHP::getInstance( true );

function logit( $var, $title="From FirePHP:" )
{
    global $firephp;
    $firephp->log( $var, $title );
}

Then I call it from anywhere (theme, plugin, core) using the function:
logit( $var_to_debug, 'The var contains:' );