When closing the active tab, the right tab always gets focus then focus the previous tab. The first time closing tab in a window, the focus always on the next tab, never focus on the previous tab.

not working with my firefox 56b12 last good version

Tab Mix Plus webextension version not ready yet.
TabBehavior has features I really wanted.

1. new tab -> right of current tab
2. active tab after tab closing -> left tab