I just don't like dealing with google's shenanigans. Like tracking, but most importantly, the potential of content manipulation. Google is already trying to stop adblocking in youtube, maybe they'll try to fuck with that. They also like to recomment to you conspiracy theories if you, say, watch even one political video. Firefox, while open source, still relies on google play to get distributed, so you never know if they're adding trackers and other shit to it. Fennec uses F-Droid to be distributed. And Fennec is actually maintained by Mozilla, the same organization that maintains Firefox. You should use Fennec, it's essentially the same as Firefox, but makes tracking harder.