Name anything Vivaldi specifically (not Chromium-wide) has done to screw over their users. I can’t name a single thing, while I can name many Anti-User things Firefox has done.
Unfortunately, open-source becomes nearly meaningless when the cost to produce a fork becomes so prohibitive and the open-source project starts acting like a for-profit company.
I can say the same thing about Librewolf, as they haven’t done anything to screw over their users either.
Vivaldi just does not have strong ad-blocking, fingerprinting protections, or privacy a preserving measures in general. Here is a comparison between some browsers: https://privacytests.org/
Here is some reading for you (if you want):
https://privacytests.org/vivaldi.html
https://avoidthehack.com/review-vivaldi-browser
