Mozilla Connect

 Not Only Vertical Tabs, but Split Screen has also now become essential for productivity & multitasking  

Waterfox looks so good in this regard.IDK why Firefox is evolving backwards in this regard. 

the ARC browser - demonstrating sidebar and split screen tabs#ARC Browser did it right. Unfortunately, Windows devices still now doesn't have support, and it's Chromium based.But, Vertical Tabs & Split-screen tabs, is literally the need of the hour.