Joomla MooTools error

I am using joomla 1.5.20 which contains mootols files i am getting error

Error: ($(b) || document).getElements is not a function
Source File: http://sitename/plugins/system/mtupgrade/mootools.js
Line: 1"开发者_如何学Python

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If you want more information about your bug you can download Firebug. That's a firefox plugin which gives you more info about javascript errors.

Firebug: https://addons.mozilla.org/nl/firefox/addon/firebug/