You mean autosave in Tree Tabs or external? My groups are internal in my extension, so if you close groups, you lose them. There is no way to restore. My extension has its own autosave, but loading autosaved session is horrible at the moment as Firefox does not provide a way to create tabs without loading them, most of the time browser is not responding and my extension just hangs, hence problems with rearranging tabs in correct groups... I'm preparing an update for Firefox 63, which will be out in 3 weeks, where creating unloaded tabs is possible (works already in Firefox beta). Most of the bugs related to it will be resolved thanks to this.

Closing the last tab in a group (if there are still tabs left) will not activate another group in the next update, but only if you close tabs from within the sidebar (with mouse, not by a shortcut or mouse gesture).