Firefox 35 includes support for the 'contains' method of ECMA 7. This breaks MooTools because it is not a protected method. Unfortunately a lot of sites are going to be broken by this, I've already tested a few including my own and jsfiddle. The specific error I see is that the Elements class does not implement the 'contains' method from the Array prototype. I've added this small change to my site and it fixes the problem.