Vivaldi is close for me on Mobile because it works really well, but I'm using the Fennec version of Firefox from F-droid currently and it's a favorite, with Vivaldi close behind. Close on mobile because it's the second closest thing for me on privacy goals, and Firefox & forks on Android are a bit buggy.
Firefox (preferably forks) on desktop fully because of the UI being open source, no slight against Vivaldi for that, I fully understand the reasoning for not doing so. I just prefer open source so I bias towards as much as possible of it when not overly difficult to do so.
I do not trust Mozilla, more open source nature of the app allows me to take trust of ownership out of the equation a bit, especially so with forks.
Fennec on F-Droid was left unpatched for months. Same with Mull, which was getting updated by its maintainers DivestOS; the updates just weren't getting into F-Droid's repos.