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" />
But your warning that you see may be trigged by one of multiple files. it might not be your HTML document. It might be something in a javascript file or css file. if you page is made of up multiples php files included together it may be only 1 of those files.
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.
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