Much of the value of Google's contacts, calendar, music player, etc. on Android is I can access the data from any browser. It's so useful I grit my teeth and share my personal info with evil Google. Firefox OS has its own HTML5 versions of those apps running locally, yet they don't run in desktop or Android Firefox. If the apps did run in every Firefox (and eventually any standards-compliant browser) and Firefox Sync securely kept the apps' data in sync (FF Sync is encrypted, so no one can spy on my personal data) then i would find it pretty compelling.
That's my 2 cents, it merely takes $20M to implement. I like Firefox, and I enjoy the sync. Having open productivity apps running in a browser fits with Mozilla's mission. I want more stuff running in a browser without spying, because it levels the playing field for Linux and could lead to a lightweight boot-to-browser environment for my phone, laptop, desktop, and tablet. Part of Google has that vision with ChromeOS, but they can't let go of the lock-in and dominance Android gave them. It's depressing seeing everyone piss all over Mozilla instead of supporting an alternative to picking a closed proprietary environment provided by a spying corporation.