Vertical tabs started long ago in Firefox. It was a feature for heavy users.
However, with time and increasing use, as well as it making lots of sense considering the aspect ratio of most computers (mobile devices - it's more a matter of habits), it just makes sense.
Recently I experienced being able to switch between vertical and top tabs using edge, which seems to have listened to users inputs in this mater.
Now, in Firefox there is a way to tweak it and eliminate the top tabs even though the vertical tabs were never incorporated into the software.
Opera, is one of my favorite, but for me at least, the option of eliminating the top tabs is a must to make Opera my "goto".