This addon idea nice but it's very bare bones and seems to have a few bugs like Runafuru said (comment below). It would be nice if you could list all the tabs and be able to open one or more of them instead of all or none. It would be also nice to be able to edit the saved tabs to remove them or re arrange them inside the group or move them from one group to the other. It would be nice to be able to see their favicon and url of the saved tabs.
BTW I wanted to contact you trough the issues section of github to have a better conversation about this but it seems like your repository does not have one it only has pull requests.