Best Tabs extension.
Would rate 5, but after the latest Firefox update, the tabs that are not in a group, are not visible at the extension, unless you add them to a group.
So, if you open a new ungrouped tab and close out of Firefox, it disappears, since it is hidden and it is not in a group.
Also, when you close out of Firefox, some tabs, sometimes gets ungrouped, and since you cannot see ungrouped tabs anymore, you got to find them from Firefox's drop down menu, since they are hidden.