One is closed source, so you can't even consider it for adoption. Google is now proclaiming its dissatisfaction with webkit, and Gecko is a 20 year old codebase now, and Google likes control.
Though considering how they fund Mozilla in the first place, if they wanted Gecko, they would just buy Mozilla. They keep the lights on anyway.
Though considering how they fund Mozilla in the first place, if they wanted Gecko, they would just buy Mozilla. They keep the lights on anyway.