"I started at Vivaldi six months ago. When I first tried the browser, one thing stood out right away: customization. While you can customize most browsers at least a little bit, no browser matches what you can do with Vivaldi out of the box. It’s quick and easy to make the browser feel yours."
Web panels embed a vertical window to the side of the web page tab you have open. So you could have an always open Twitter or Instagram feed there right next to the page. A tip from the dev is you can try the mobile view to ensure it displays nicely.
Not to be missed either are the Notes and Tab stacking functionalities.