The usage of Safari is 3-5x that of Firefox.
It's Safari and Firefox as the last competing browsers standing, and Firefox is increasingly irrelevant. Whatever your opinion of Safari is, soon it will be the only browser trying to resist Chrome. And judging by the amount of new "standards" Chrome ships enabled by default with each release, Google couldn't care less about Safari either.
Firefox doesn't come with OS and hardware to keep it around.
So they're very aggressive to introducing "app-like" APIs to Chrome even often at the expensive of user experience, performance and security.
As you said, their very goal is turning the web into ChromeOS. And the web is everywhere.
Honestly, that's NOT a good thing for the future of either the web or applications.