Very good job, so far it is the best replacement I have found, and considering the limitations we have to work with now a worthy replacement of what used to be. When I found out Tab Groups no longer worked I considered making my own version, especially after I tried a couple others, thankfully I found your version and it saved me the trouble.

There are a few things I think are missing, but they might be limitations of WebExtensions also, I haven't done any work in WebExtensions to know. The most important for me would be just the ability to organize the order the groups show up. The second most important would be the ability to close out tabs without actually opening the tab group.