I'm using this in conjunction with the 'Tree Style Tab' extension because that one adds some new tab behavior and not all the options to change it to my liking. Hence some other extension was needed that got me back the functionality I had with 'Tab Mix Plus' in versions <=56 of the browser (After Close, Change to Last Active Tab). It also does not appear to interfere with the grouped tree behavior when opening tabs. All in all, I'm happy with it.

Edit: if there was one thing to criticize then it's to make the extension page on AMO more user-friendly. Add a more detailed description, maybe create a graphic/modified screenshot to illustrate the changes that are possible. I don't need any of this, but for the average user it would undoubtedly help