What primarily drew me to Vivaldi browser initially was that it was one of the very few Chromium based browsers with a good implementation of tabs in the side panel instead of using valuable vertical real estate on wide monitors. That was of course before I discovered that all those good parts of Vivaldi were closed source, and I stopped using it as they were not sharing their improvements after building on top of the free open source they received in the first place.
Be that as it may, the point is that with 1920 x 1080 screens there is often a lot of wasted space on the sides, so it just makes sense to be able to have your tabs on the side. Although Firefox does not do this by default, at least various extensions can do this quite well (unlike on Chromium based browsers where the extensions are not very stable).
See Google Chrome’s side panel is a new UI for your reading list and bookmarks
A nice new button