While not exactly FLOSS, Vivaldi gives us its source code, i.e. most of it. But being (partialy) closed source doesn’t necessarily means it is less secure and/or less private. I understand they want to keep important parts of their code for themselves, as it it something that makes difference. When it comes to features, Vivaldi is one of the best browsers out there, alongside Opera (which is privacy nightmare, that’s the reason why Vivaldi is created).
Out-of-box, Vivaldi is not perfect privacy-oriented browser. Mainly due to default bookmarks they put. But they need to make money somehow, just as FF use Google as def. search engine. But it can be tweaked quite well. Maybe not as good as un-googled Crhomium, but as I mentioned, its features are way better. So it is compromise between two. As I use FF as primary browser, plus Tor for some other stuff, Vivaldi is great option when I need chromium based browser