Why? They allowed proprietary extensions (e.g. Flash) from the start. I don't like it, but I don't see how it represents a loss of their way. Mozilla was never GNU.
Big difference between an extension and being integrated into the browser. It's directly analogous to the difference between your OS being closed source and your OS being able to run closed source programs. The former is a liability; the latter is an ability that you grant to users to use the system the way they want.