Simple Tab Groups is "exactly" what I was looking for to replace the old Tab Group Manager! I delayed upgrading from the old version of Firefox for a long time because I didn't want to lose the ability to group tabs. I tried several other tab managers first, but they didn't do what I wanted. STG is very intuitive, very functional, and easy to learn. I can't say enough good things about it! The only thing it lacks, compared to the old Tab Group Manager, is the ability to resize groups in the "Manage Groups" display, but this is a minor issue, and in some ways the ability to scroll down for additional groups is an improvement over Tab Group Manager, which couldn't scroll and would get very crowded with a lot of tabs and groups.