What is this?
The built-in dialer is conventional.
Apps ... sort of
Firefox's apps are actually self-contained websites or web apps.
This is what the relatively simple texting app looks like.
The operating system comes with a built-in music app that scans on-board media for files it can access and play.
Naturally, Firefox OS comes with the Firefox web browser.
Firefox's marketplace is much like Apple's App Store.
This is what the Twitter web app looks like running on Firefox OS.