For a while now, performance has not been an issue for me to not use Firefox. It's the no support for the multiline tabs and/or tab groups. Both features which were previously possible in Firefox. This is pretty much a blocker for me and they don't seem to have any plan to fix this.
Are there reasons that the "Simple Tab Groups" extension isn't suitable for you? It seems to be well-maintained, cleverly uses Firefox's "hide tabs" API to keep a single window behind-the-scenes, and has integration with lots of other features like Firefox's containers. I think it's an indirect descendent of the old Tab Groups extension when it was spun out of Firefox.