Re: Vivaldi is great
The Tree Tabs (I don't remember the exact name) extension for Firefox was better, but it's impossible to enable it now without also having the standard tabs across the top of the window. Which is monumentally retarded.
Vivaldi's side tabs are ok, but still quite annoying at times. (Trying to actually find the single pixel line to stack a tab is irritating as hell, and the whole concept of tab stacking leaves much to be desired - especially when you're trying to find a tab again.)