I really want to like Vivaldi. It's the only browser (afaik) that supports tabs on the side. Right now I use "Tree Style Tabs" on Firefox with some custom CSS to get rid of the original tab bar.

Unfortunately I get a massive delay when fullscreening youtube videos in Vivaldi. The video first maximizes to the left half of my screen, and only then expands to the whole thing. Sadly a dealbreaker. FWIW its a little faster if I have the browser already maximized.