Falling behind on the browser core makes users leave Firefox because the one non-negotiable requirement users have is that the browser works on the websites they need it to work on. Literally nothing else Mozilla tries to do to diversify their revenue streams will matter if the browser they're trying to build it on top of is not sound and functional.

If they weren't slipping on the core browser as much as they have been, there wouldn't be nearly as many shouts as there are today when they instead spend resources on chasing the latest fad.