This is a sorely needed add-on and I am grateful that it is in development. I had been holding off coming back to Firefox after the Tab Groups add-on was unable to function with the Quantum version of FF. Even though I am now on board with FFQ I do have a few things in my wishlist:
1. when changing tab groups Panorama View always opens the rightmost tab, I order my tab usage from Left to Right and would like the ability to select which end tab PV opens on a Group change.
2. I have to use a separate add-on to have PV not change windows when I close the last tab on the right, my thought is this should be incorporated in PV.
3. A method of moving a tab from one group to another group via a right click on the tab, at the moment a move to a new window is available but not what I had in mind as far as the work I do.
4. Perhaps display the name of the tab group on the navigation toolbar instead of just the icon. (added 11/28/18)

With that being said I feel PV is very useful and it does fill the void for me. The functioning of it in it's current state is quite good and seems stable. I have nothing but praise for the effort on this project, keep up the good work.