Vivaldi's pretty good feature-wise. It's got tab workspaces and stacking, detailed themes, mouse gestures, menu customisations, tab tiling, it's basically where a lot of the Firefox power-users have gone. It's a solid browser. That being said, a lot of the Firefox fanboyism is prevalent here so you're going to make a few people angry if you suggest anything that's based on Chromium. Just use whatever you see fit - I personally don't care too much about open-source vs. proprietary, I use Steam for crying out loud.

That being said, I've also gotta shill Pulse. I don't like the way Mozilla is running Firefox so I'm steering away from that, luckily Pulse is forked from Firefox. I'm concerned about Mozilla treating userChrome.css as a legacy feature, so having native vertical tabs is great, and a generally minimalist UI makes it a very clean looking browser to use. With Simple Tab Groups, the only extension I've installed for extra functionality with the browser itself, I've basically got my Vivaldi replacement, though If Mozilla breaks STG I'm moving my ass back to Vivaldi.