Great extension, thanks! We're slowly clawing back some of the functionality we used to enjoy in the pre-FF57 Tab Mix Plus.
I love that you can use shortcut keys, although it took me a long time to figure out how to do this on the options page (you first have to define the shortcut key at the bottom, and then select the shortcut key in the "After closing the current tab, try selecting in this order" section).
I have "last accessed tab" as my first preference and Ctrl+Q bound to "right tab", so if I want to close the tab and select the last accessed tab, I press Ctrl+W (the default key for closing a tab), but if I want to close the tab and select the right tab, I press Ctrl+Q. This works really well. (I would have preferred Ctrl+E, but that does the same as Ctrl+L and can't be overridden, it seems.)