Scam.
After I reported this addon to Mozilla they removed the link to their website, which was a Casino gambling website.
I don't understand why Mozilla still has not taken down the whole addon, though.
It is clearly a fake of the real (and amazing) "Simple Tab Groups" addon by Drive4ik, which I have been using for years now.
Also, why does Mozilla allow someone to use the exact same name of another addon is beyond me.
This addon even specifies "simple-tab-groups" as the name of the author, to trick better, which also should not be allowed.
You can only spot it is an imitation if you look at the slightly different URL, and if you look at the number of users.
But who does it?