I'd still be seriously concerned about how few eyeballs they get with regards to security---Google has entire fleets of machines dedicated to fuzzing every line

I'd still be seriously concerned about how few eyeballs they get with regards to security---Google has entire fleets of machines dedicated to fuzzing every line of Chromium code and both Chrome and Firefox use advanced sandboxing to make it so that a single exploit in the JavaScript engine wouldn't be able to result in user-level remote code execution. I'm sure they're both probably fine (if nothing else from security by obscurity) but modern browsers are complicated pieces of software that require a lot of engineering to keep secure.