Firefox and Chrome don't do "move fast and break things"-style continuous deployment, can't push updates to a user seamlessly (i.e. without restarting the program), and can be downloaded and compiled on platforms that Mozilla and Google don't want to officially support.
Native delivery of a monolithic browser based on an open-source codebase is a fixed problem. Trying to do the same with a website, using current techniques, would cause issues to both their current workflow and my expectations as a user that websites don't currently have.
I'm not saying that it's impossible to do, I'm just saying that it's not a good fit for current trends in web development, and I'm not convinced that it would be great for the users either.