Provides ability to go back/forward through tabs. In other words, when you cannot go back anymore by the "Back" button, then this Firefox will focus to the owner (opener) tab of the current tab (and, the "old" current tab is possibly closed.) For "Forward" button, Firefox focuses to the suitable "next" tab if possible.
Those navigations work more intelligently if Tree Style Tab is installed.
If you see any problem, see FAQ at first. If there is no solution, post a report to the issue tracker on the GitHub please.