I was a big fan of Opera back before there was a Firefox browser, back when it was all Internet Explorer as far as the eye could see. Opera was an interesting thing, and was also a source of real and sustained innovation. Tabbed browsing was their idea, as was mouse gestures and a few other fancy things.
Opera as a company has gone weird since then, but a few of the original people have splintered off and created Vivaldi , and I kind of love it. it's a deeply nerd-centric browser. Right now I'm writing this post in a sidebar, which lets me browse sites I'm wanting to reference in the main window. Add in things like “tab stacking” and a command palette, and —a signature Opera feature— a built in mail client, and you have a delightfully non-mainstream browser that I have fallen in love with more than once.
I tend to fall out of love with it because of compatibility issues with some of the sites I have to use for work, but hopefully as Vivaldi advances, and as web masters within my organization update their sites, that will be less of a problem.