This is a very useful extension. I wouldn't say it's simple, but it is powerful.

It's useful to be able to move tabs into groups based on their URLs; I use this to separate different projects so that I can switch context between them.

It doesn't seem to play well with other extensions that automatically close tabs in the background, though; for example Dustman. After a tab is auto-closed, when I switch to that group, STG says “Invalid tab” and “Background not inited”. After that, STG doesn't work. If I disable and re-enable it, the toolbar button never finishes loading. Restarting Firefox seems to fix it. (I think this may be the same issue as #212 on GitHub.)

Edit 2018-07-20: Thanks, I've updated to 3.0.10. Let's see how it goes!

Edit 2018-07-23: I'm still seeing “Invalid tab” notifications that stop STG from loading the group, but I haven't yet seen a “Background not inited” one, and STG starts working properly again when I reload it from the menu.