I have forked this plugin on to GitHub to incorporate existing and future bug fixes. Please feel free to report issues here:
https://github.com/msigley/firephp-for-wordpress
Once in a while I will see an error for ob_flush being called when there is no buffer set. The fix is as follows.
public function onFooter()
{
if ( 0 < ob_get_level() ) {
ob_flush();
}
}