EDIT: Looking at bugzilla, looks like the author of Tab Groups decided he was going to put together a report about a year ago, and then never did, so Mozilla doesn't even have any documentation of what functionality would be necessary to implement it, and there's currently nobody interested in implementing it.
Also, Mozilla knows perfectly well which functionality is needed to implement Tab Groups because it was an actual feature of a browser before they decided "it should be a plugin". And now it's impossible to even write a plugin for it.
It's one or the other.
Either the API gives very deep control, and it must evolve as the browser evolves, or it does not, and it can stay stable.
Experience says that Firefox was on the wrong side here by offering complete access, thereby either blocking the browser to evolve, or forcing add-on authors on a compatibility treadmill, effectively getting the worst of both worlds.
Maybe it's possible to do better. But we haven't managed to do so, and neither has our competition. So it's probably not easy.