CKEditor sheets.addRule not working - js exception in FireFox

Hi,

for some reason the CKEditor is now throwing me the exception about missing function addRule.

I've dumped to console the objects and there really isn't any function with this name.

But it works OK in Chrome 37, IE 11, Safari 5.1.7; only FireFox 32 has this strange problem.

Does anybody know where to start finding the problem?? 

Thanks