That helps: (by rk)
EDIT. For those having similar issues. I think the trick is to open about:config and set closeWindowWithLastTab to false.

Simple and effective.

I tried more group tabs extensions, but this is the best solution for people who were using the old Tab groups manager which worked before Firefox Quantum. Many thanks!

Najlepsza!

Awesome add-on

Pretty great add-on, just wish I could open a group and have my currently open tabs merge into it.

Exceeds expectations! Lots of well thought-out functionality. Browsing is 100x better, easier, and less distracting when you don't have to scan through a huge list of tabs OR clumsily switch between resource-hungry windows to find the tabs that are relevant for an activity. Tab Groups for the win!

Best of all, the execution is so elegant. It's easy to view your tabs from almost any menu, move them between groups, and customize your groups. We're given practical options like unloading tab groups for when they're not in use. Well done.

Great extension. Wish there was a way to short tabs within a group and also resize groups in manage groups