UserAgent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0

Steps to reproduce the problem:
Code:

  var testArray = [];
  for(var i=0; i<11; i++){
	testArray.push({index:i});
  }

  console.log(testArray[0].index,
           testArray[Math.floor(testArray.length/2)].index);

  testArray.sort();
  console.log(testArray[0].index,
           testArray[Math.floor(testArray.length/2)].index);

Result in Chrome:

  0 5
  5 1

Result in Firefox:

  0 5
  0 5

What is the expected behavior?
All array elements are compared as strings "[object Object]". Consequently their location in the array does not change.

What went wrong?
Why swap the elements at indices '1' and 'Math.floor(testArray.length/2)'

Did this work before? No 

Chrome version: 51.0.2704.103  Channel: stable
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
Flash Version: