This issue concerns me for it appears to be related to a fix I added to allow the ClientAPI to work well with other javascript frameworks.  I believe I did not have enough checks in place, for it appears that FireFox defines an Element object by default.

Line 908 of the dnn.js file reads as follows.

if (ctl != null && typeof(Element) != 'undefined') //if prototype loaded, we must extend the object

It Should read

if (ctl != null && typeof(Element) != 'undefined' && typeof(Element.extend) != 'undefined') //if prototype loaded, we must extend the object