Just a matter of convenience... I believe on a large screen it makes more sense to have fresher tabs closer to the top, the most active area of the browser (all the controls are there), as less eye movement between them is required.
I didn't even realize Firefox has vertical tabs until yesterday because Chrome launched it 😂 It's pretty great but I just wanted to add a +1 because my eyes and muscle memory have me looking to the top for the new tab button, but it should be up to each individual user preference