It is really not convenient. I cannot use multiple groups at once, I cannot use pages that are not grouped. Group switching is very slow, 3 clicks are needed compared to Chrome's 1 click show/hide feature. The menu system is VERY un-intuitive.

So I have about 200 tabs open usually. Is this a bad habit? I don't know, I just have different areas of interest and they just seem to accumulate.
Anyhow, I tried all the 'tab management' add-ons available, and this is the only one that works and made sense to me. I have about eight different groups, and navigation is simple.
User Tip: when you're on the 'Manage Groups' page, if you copy the web address, and then paste it in Firefox Settings > Home > Homepage, under 'Custom URL', then when you click on the Home button it brings you to the Manage Groups page. That is what make it work so well.
There was another tab management add-on which I really wanted to work, Tab Stash, but couldn't understand it, found it unintuitive (gonna write to the Dev), so this one wins!
Fantastic, many thanks Dev!

"Pinned tabs are not supported".

Upd. After enabling browser.tabs.groups.enabled, the need for a separate extension disappeared.

One of the good alternatives to Vivaldi's tab stack feature.

Excellent replacement for the old Panorama. The only... doubt is that last update was about a year ago. Is it still supported?

Really good for organizing tabs, but when you have many tabs stashed or organized it will be very VERY slow to show them when you go to settings. I had to remove the extension because of that.