Ok, the real best thing is that this is the best yet for privacy and freedom, better than any other phone out there.
Also, storing all your data on someone else's server is obviously the best thing yet for privacy and freedom.
I already stored all my data on Google server thanks to Android. And I use google maps and google now so they know more about me than my wife does.
In fact most of my apps on android and iOS like linkedin, facebook, twitter, instagram, mint etc are storing my data on their server.
I do think it's worth thinking through privacy/security/freedom implications, here is my opinion (not necessarily my employer's):
All web apps can of course operate completely in offline mode (as well as privileged FirefoxOS apps[1], which have access to APIs to use more of the device features and are the bigger concern here) - all of the apps that ship with the phone do operate and store data offline, for instance (e.g. you can have a local calender, it's not stored on Mozilla's servers).
Knowing whether you can trust an app to treat your data as they say they will is not anything unique to the platform (who/where do you get it from, etc). Mozilla is very serious about this type of thing (I work full-time on various Mozilla server projects so I constantly experience first-hand how important this is to everyone, e.g. application design, handling crash reports, etc.)
However there does need to be a balance between the needs/desires of the platform provider (including built-in apps), third-party app developers and the end user - developers of third-party apps should have the ability to make choices about where data is stored, privacy policies and so on, and users should be able to install the apps that they want to.
[1] - https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS/...
My point was that the assertion that this OS is somehow superior to android or iOS in terms of freedom or privacy is completely unfounded.
You do not need to search every time, this type of app can be saved to the home screen. You could think of it like a bookmark to a tab that will run chromeless in it's own process (so no access to cookies etc. in your main browser process). For all practical purposes it is a low-privileged "hosted" app like any other - has it's own icon, possible to terminate, sandboxed and so does not share data with other apps.
FirefoxOS has a feature that allows you search and browse a set of these apps provided by everything.me but it's also possible to do this directly from the built-in browser app.
My point was not to attack Firefox OS, but to point out that these home screen apps are indistinguishable from "installed" apps.