After the call for native tabs on large screen mobile, to me the next logical call is for vertical tabs, as there are added right now to the desktop version.
Since what is really called for is a closer resemblance of the large screen mobile and the desktop version of Firefox.
(I also want to point out that on mobile phone the tab menu is basically vertical tabs, if this menu of phone would appear as a sidebar for large screen device, it would add a lot in my view.)