After installing you can configure this extension by clicking Preferences of it in the Add-ons Manager. The buttons represent the order of preference: e.g. if "left tab" is at the top and "last accessed tab" is the second, after closing the current tab it will try to select the left tab, and then the last accessed tab (if there is no tab on the left). Click the up and down icons to modify the order.
The "opener tab" option uses an upcoming Firefox feature and doesn't work in the current Firefox version (2017-08-14). See this bug for technical details.