This isn't the gotcha you think it is. Every time I try Vivaldi I am right back at Firefox and I am surely not alone. I have never understood the obsession with tree style tabs or vertical tabs. I don't need to customise my browser at all and I like supporting engine diversity.
> I don't need to customise my browser
Ok, but not every use case is so primitive? I do need my custom shortcuts and what not, so it is exactly the correct "gotcha" I think it is even if that's beyond your understanding.
went from Firefox to Vivaldi, never looked back for many years
on Android phone tried many, most recently was using Kiwi Browser, then for some time Firefox until they fucked up UI, so moved to Cromite, though my phone broke (never buy Google Pixel again, first broken phone after 15 years with smartphones and various brands including very low budget), so now I am on my old phone which for some reason doesn't support Cromite, so I am back at Firefox temporarily
The motivation for vertical tabs is pretty straightforward, screens are mostly wider than they are tall, browsers are often used in fullscreen mode, yet much of the web does not use much of the screen's width. So it's a better use of screen space to put tabs on the side than on top.