That's actually bad news, because it will severely limited with no nesting. Sidebery or TST won't be replaced with this.
If this means I can remove my firefox CSS that hides the horizontal tab bar at the top, and instead have it hidden automatically whenever I have Sidebery open, then this is an advance
This is absolutely not that. Another thing that I miss a lot from early-mid days of browsing is "Close tabs opened by this tab" button.