At a minimum, I am going to need either Firefox or Chrome because the employee service center web site for the company I retired and am drawing a small pension from requires either Firefox, Chrome, or IE. There insistence on allowing IE as an acceptable browser but not Edge or Opera is humurous.
I am currently torn between Opera and Vivaldi. The interesting thing is the above employee web site recognizes Opera by name, but not Vivaldi. Both work with the ScriptSafe extension, an absolute must have (with one possible hitch with Vivaldi I'll note below).
Opera is faster than Vivaldi, and it's faster than every other browser I have tested. I don't have a lot of look and feel customization with Firefox, but I do wish I could change a few things with Opera (like replacing the Start Page button with a Homepage button). Despite the relatively sterile theming, browsing with Opera is better than with the long-used and tailored Firefox.
I like that I can customize Vivaldi, but it seems like there are too many customization options. I wonder if this is part of the reason Vivaldi tends to be a bit slower than Opera and Chrome, but actually on par with Firefox (my version with extensions). I think the built-in ad blocker for Opera helps with its browsing speed. I haven't determined whether the built-in ad blocker is better, same, or worse than the uBlock Origin extension.
It's not just Vivaldi's main browser that is slower than Opera. When I access the ScriptSafe menu to make changes, Opera refreshes immediately upon exiting ScriptSafe. Vivaldi does not with this extension. I have to refresh the web page for any changes to take effect with Vivaldi. I don't know if this is typical with Chrome related browsers with the ScriptSafe extension.
But I do like Vivaldi can use just about any extension that works with Chrome. The same cannot be said about Opera. But even this is not a huge issue. Other than look and feel, I figure there's about 5-6 extensions I would want, and they are available for both Opera and Vivaldi.
As stated before, being bought out by a Chinese company last year is my main concern about Opera. Others seem to be put off by the propriety nature of their OS, especially going forward. Opera has been around long enough to where the latter doesn't bother me.
I'll keep it on the machine and check it occasionally to watch its evolution; but I have removed it from consideration as a Firefox replacement.