I was confused at first, didn't really understand how it worked.

The tabs are basically grouped by window, which is ok i guess but not what i was expecting.

You make a group, and if that group is active then any tab you open in firefox will be added to that group. any tab you close will be removed from the group.

If you activate a different group, your current tabs will be closed (and saved) and the new group tabs will be opened. In other words, you can't have two groups open at once.

It was unintuitive to figure out, but easy to use after figuring it out. However the functionality is not exactly what i want, the tab grouping in chrome is way better than this.