Ah, I missed that you're talking about the Firefox Error Console. We don't influence the output of that console and it's unrelated to Firebug.
The error message already gives a hint for what to do to avoid it:
"The character encoding of the page must to be declared in the document or in the transfer protocol."
So either set a <meta> tag like
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> or add an HTTP header like
Content-Type: text/html; charset=utf-8.Or if you really just want to filter that message out, I suggest you install
Console².
Sebastian