As tab mix plus no longer works, this is certainly a welcome app. It does what it says, it transfers focus to the tab on the left when closing your current tab - But! - it does not prevent the tab on the right from loading, as tab mix plus did! So, even if not focused, your youtube video, or whatever, will play merrily away, even if you are not focusing on it.

If the developers can get Firefox to 'not load' the tab on the right of the closed tab, before it quickly focuses on the tab to the left, then this app will be perfect!

Great extension! Work perfect

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.