no, i'm not printing a "<br/>" anywhere in the output.
But, interesting side effect. Upon trying to create a test case for you, I'm getting the same error with JUST the following script.
<?php
ob_start();
echo 'line 4';
// setup FirePHP output
define('INSIGHT_CONFIG_PATH', FS_ROOT.'includes/FirePHP/package.json');
require_once(FS_ROOT.'includes/FirePHP/FirePHP/Init.php');
$inspector = FirePHP::to('page');
$console = $inspector->console();
echo 'line 13';
$console->log('foo');
echo 'line 17';
?>
which makes no sense to me, since I'm not doing anything but doing a simple log.