Uhm, no? https://vivaldi.com/source/

Only part of their source is available.

> Of the three layers, only the UI layer is closed-source. ... The Vivaldi UI is truly what makes the browser unique. As such, it is our most valuable asset in terms of code.

https://vivaldi.com/blog/vivaldi-browser-open-source/

Being able to contribute to ui code, or to customize ui without going through the minified code would be the main benefit of browser being open source. For discouraging forks, using a non-permissive license for js code, would be as effective as minification.

Which is kinda funny since their UI is half-done and sluggish.

And yet it provides features that I desperately would want in Firefox, such as tab tiling. Really miss that from Vivaldi.