I wouldn't call Firefox stagnant. It's just got E10S (multiprocess). The Quantum project promises to gradually improve both speed and stability (and consequentially security). They are working on replacing the traditional Firefox extensions archetecture with WebExtensions. Fennec (Firefox on Android) is an underappreciated gem.
I'm playing with Vivaldi now. It's nice, it's fast, there's a fair amount of UI innovation. But underneath, it's just Blink, so it's hard for me to get that excited about it.