I have been using vertical tabs since the mid 90s - OmniWeb on NeXT then OSX. Unfortunately that stopped being usable in the late 200s then I used Firefox extension TreeStyleTabs - I did not know that Mozilla did vertical tabs. Chrome also had vertical tabs then but the bug tracker is hilarious in that the authors really did not understand its use and eventually removed it.
After 2016 then Vivaldi and in the last couple of years Safari (there was a horrible admin 10 years ago)
After 2016 then Vivaldi and in the last couple of years Safari (there was a horrible admin 10 years ago)