There are definitely upsides to using Firefox. I do find that the currently available ad-blocking extensions work better on a certain popular video hosting site. I still use Firefox so I can listen to use that site to listen to music without a 17 minute infomercial playing in the middle of every song. But I don't think its fair to say that Vivaldi is Chromium. Vivaldi uses a fork of Chromium's Blink rendering engine. I'm not really super worried about that aspect of a browser. Every browser is built on one of like three engines and they're all decades old. Mozilla was developing Servo as a next gen but it was written in Rust, so it died when Rust got axed. I'm more concerned with data transfer security, telemetry and UI. Vivaldi wins in all those categories IMO.