The only problem which Mozilla didn't think about or hasn't given much value to is the huge number of add-ons that are going to die or just stop working when they completely switch to WebExtensions (as there's just no replacement to the previous API in them). One of the Firefox's USPs was its addons which could modify the browser behavior and UI in any manner. It's all going to go, and Mozilla is moving closer to bec…
It makes use of the new Containers experiment to provide a similar tab organization mechanism as TabGroups, and presents them in a vertical sidebar which IMO makes for a much saner workflow for heavier tab users. The identity encapsulation that comes by default with Containers might not be what you're looking for in a pure tab management system though, but as Sea Containers shows, some variant of tab grouping can definitely be implemented in WebExtensions.