Good Addon. Not sure if the means of always closing tabs when switching groups is a generall problem with firefox 54+. Ofc it would be nice if they could stay opened. Let's see how it goes in time.
One other bug is, when I close the browser and reopen it: the tabs from my session reopen without a problem (they are not lost) BUT they are in no group anymore. Would be nice if the addon recognizes this was the old group and uses it. Workaround (and general nice feature) would be: when you right click tabs you can move them to group, so having a second option, when clicking on a tab to "Move all open tabs to group ..." would help a lot :) so I at least dont have to click on every single Tab of a window. Good work so far!