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.
Small development I thought was worth mentioning here: Vivaldi now gives you the option to move the new tab button to the top! You just right-click on the tab strip, click "Customize Toolbar" and then drag the new tab button to the top. The only issue is that the new tabs still open at the bottom of the list but I think they said they are working on that. Hopefully Firefox devs will feel more comfortable doing this now that Vivaldi has.
Yes, please make it an option to have new tabs appear at the top rather than at the bottom. I recently switched to vertical tabs and I like them very much except for this one thing.
The new sidebar is a great addition, but ordering the tabs from top to bottom, with the newest down the window, although it fits the natural reading direction, is a little bit unnatural to me. I'd rather have the tabs (and the tabs icons, obviously) pile up in reverse order, from bottom to top, with the newer ones well in my line of sight at the top, rather than at the bottom of the screen. It would come to having a simple boolean data type field (...tabs.order... true/false) in the about:config and/or an additional parameter in the Sidebar Settings.