Firefox sorting behaves different than in other browsers

[jschaefer-workmatrix]

Shuffle version
6.1.0

Describe the bug
Sorting bevahior differs in firefox.
When using sort with a custom compare function the sorting breaks in firefox due to this fact.

Reproduction link
A minimal JSBin, JSFiddle, Codepen, or a GitHub repository that can reproduce the bug. You can use this "Shuffle with Images" template on CodePen to get started: http://codepen.io/pen?template=qrjOpX

To Reproduce
Steps to reproduce the behavior:

1. Visit the codepen with any browser other than firefox
2. click on an item and see the sorting work out
3. Do the same in firefox
4. you will see, that the sorting completely differs