I think this problem can be found with older version of MySQL DB then 4.x
I solved this problem in /components/com_fireboard/template/default/post.php on line 220 :
modify in SQL query JOIN to LEFT JOIN
$database->setQuery("SELECT id FROM #__fb_messages LEFT JOIN #__fb_messages_text ON id=mesid WHERE userid={$my->id} AND name='$fb_authorname' AND email='$email' AND subject='$subject' AND ip='$ip' AND message='$message' AND time>='$duplicatetimewindow'"

;
I tested it on multiple sites and it seems to be working good
