LOVE this extension!
Yup, it's simple, and it's great for keeping organized!

I have a group for "general", one for school, and one each for whatever games I might be into. (Right now Satisfactory and BattleBit.)
Super amazingly convenient to have these tab groups ready to go. They are much more flexible and easy to adjust than a bookmarks folder. Just right click a tab and send it to a group, or use the pop-up, or the manager.

You can even choose to hibernate tabs/groups (or not), but they'll still be "open" for later use. You can have a tab loaded and running some demanding applet, but still get it out of the tab bar so you can focus on something else momentarily. It's amazing.

Some of these functions are available in other extensions like Sidebery, but again I like this one for the Simple aspect. I use this in tandem with Tab Center Reborn for a (again, simple) vertical tabs sidebar, and now my toolkit of tab management is versatile yet lightweight, and still in dark mode!

I nearly forgot that sadly I'm here to comment on a pet peeve. The tab overflow icon, that little arrow managed by the about:config preference browser.tabs.tabmanager.enabled, is for some reason inescapable when using this extension, unless you are in the "other tabs" misc group which defeats the purpose.
Something to do with the behavior of the arrow: it is only supposed to appear if you have many tabs, but with this extension I can close all my tabs and just having the tab groups active even when empty shows the arrow. Small irk, very silly for me to be frustrated by, but there you go.
I don't know if I could possibly work around this using the extension's settings, I haven't tried, and I don't know if it could be patched by the extension dev to behave differently. So it's definitely still a 5 star extension. I love all the other stuff it has going for it.

Thanks Drive4ik for the great extension!!

BTW Mr. Dev, if you're reading, any chance for a feature like "automatically discard previous group when switching groups"? Or maybe a button that will switch to a group and discard other groups at the same time? My machine is not super strong and I run multiple apps at once, so it's usually best for me to hibernate inactive groups, and it's tedious to right click -> "discard other groups" all the time. Thanks!