This error comes from the javascript code that handles errors.
Next step:
open /com_fireboard/sources/fb_bb.js.php
find:
formmessage=document.postform.message.value;
if ((formmessage.length||1)) {
alert("You forgot to enter a message."

;
submitme=0;
}
Comment this section out. Now, if a user forgets to enter a message, the error will be returned after they navigate away from the page--inconvenient but not unbearable.
There's a way to modify the js code to give errors for the tinyMCE message area, but I haven't figured it out yet. Will post when I do. Anyway, that's not essential for now.