In your HTML it is a good pratice to provide the encoding like using the following meta like this for example:
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
Note: if any line between
<head>and this meta line (Even if it’s a
commented note) problem will remain, so make sure you added meta line
I dont think this error has anything to do with mootools. you see this message in your firefox console window. not mootools script.
maybe you simply need to re-save your html pages using a code editor that lets you specify the correct character encoding.