Yes, Vivaldi isn't fullFOSS, because 5% of the script of the unique UI is proprietary of Vivaldi, but it's 100% auditable and even moddeable by the user, they even show how to do it in its community. Edge and Chrome would fork it in the same moment when Vivaldi make it OpenSource, killing all other Chromium and Vivaldi itself. Maybe in the future it will go full OpenSource, there are still intern debates about it. The sense of OpenSource is to be capable to collaborate in new products, but with almost 100 browsers and forks in the market, this value is pretty debatable. For the user is more important the ethics of the company respect the user, in this case a european, employee-owned cooperative, which is given with a full transparency in all it's services included in the account (mail, calendar, feed, blog, the Vivaldi Mastodon instance, e2ee sync in own server, etc.).