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I get a
"navigator.plugins['Shockwave Flash'] is undefined"
in mootools 1.4.5 on Firefox 18 when I don't have flash player installed.
This happens on several of our sites running MooTools.
So this bug is not what it seems. The error only shows up if another bug exists in the code. ie. inserting null element into DOM using grab.
In the next version the flash detection will be removed by default (it's only in the 1.4 compat).