I used Vivaldi on and off over the past few years, but I've been using FF full time since late last year. Vivaldi eventually starting slowing way down for some reason. Startup took nearly 10 seconds, when it used to be < 3. Not sure what did it. It may be due to the chrome (UI) as it's a very heavy custom CSS solution.
Also their address bar behavior was way different than Chrome or FF, and it kept messing me up.
It might be better now, they've had many updates since I last used it. Might give it another go, now that FF is doing this stuff.