I use Firefox except for when something's broken, in which case I use Chrome. It's rare but it does happen.
I'm not a huge fan of Firefox, but it's both open-source and fairly trustworthy. Lots of browsers are one or the other but I think Firefox might be the only bigger one that's both. If Vivaldi was open-source, there would be a good chance of me using it though since it seems like a pretty neat browser.