Even if this is catch-up with respect with the other browser, I think that this mean that there would finally be a non-hacky way to disable the tab bar (i.e. a toggle rather than something that on userChrome.css).

I'm perfectly happy to have only basic vertical tab functionality on vanilla Firefox and Tree Style Tabs or Sideberry for power users. Presumably there would also be API that makes the life of piro (main dev of TST) and mbnuqw (main dev of Sideberry) easier ?