I think he meant a few checkboxes, for choosing things like round corners etc. which should not be too hard. Really necessary?
Maybe not, but you should realize, that you have a shrinking core group of users, while liked Firefox because they could modify it the way they wanted - and therefore did not liked it, when frequently changes broke their setup.
Which is unfortunately an inherent problem with our legacy customization approach.
And it is clearly not something the vast majority of users want, if they left Firefox for another browser that appears to them less customizable but more stable. We have run numerous user studies about what users actually care for and they have all informed what Firefox is going to be in the future.
So does not particularly well thought out criticism.
There's also shrinking core group of users, who liked Firefox before it fell back on many areas, crippled by having to maintain compatibility with add-ons that relied on internal workings. We realize that very well.
Configurability comes with an exponential testing and maintenance burden. So no, it's not just adding a few checkboxes. It's what happens with those afterwards.
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.
Yawn. What did we do this time? Make it faster? Follow platform guidelines or modernize the design style? Adopt an extension model that can be combined with replacing the browser internals? Use rounded tabs? No wait, scratch that, they're square again.