Did exactly what it was supposed to. One of the functions I used from Tab Mix Plus which is being nixed in FF57. Thanks!

Edit March 2018: It still works, but I have to disable and then re-enable the addon each time I start Firefox. ANd sometimes it hovers on the right window before switching to the left.
I mentioned this on their github site but no response in months. Shame. 5 star down to 2.