First of all: A BIG THANK YOU! This is looking very good! FF after 56 was a big regression!
Tab groups should be part(optionally or not) of every FF, like tabs are. I mean if you have files, you have to have folders/directories!
In order to be 100% helpful, tabs, have to have 4 important things:
1. Free arrange and rename the tab groups;
2. Saving the session at will and automatically(every hour or so) before FF crashes or is closed normally;
3. Possibility to move tabs from one group to another.
4. A "tab group bar" above the tab bar, with a button to hide/show the "tab group bar".
I guess you have already covered most of this and I hope you will do your best for the others!
EDIT : take a look at this: https://postimg.cc/SJhgfGdn
for clarification about paragraph 4. See that green T to the right of the tab bar....that is the button with which left click show/hide the "tab group bar".