As for the manifest file, you can get by with as little as:
{
"name":"Example",
"developer": {
"name":"Developer",
"url":"https://example.com"
},
"description":"My Description",
"icons":{
"128":"/img/icon.png"
},
"launch_path": "/",
"installs_allowed_from": ["*"]
}
It is relatively easy to get the app installed from your own site, if you choose to ignore the Marketplace entirely (our code does it in ~80 lines of CoffeeScript) as seen here: http://www.youtube.com/watch?v=FMxeK1_xY98
One thing to keep in mind is that the window chrome which will house your application does not have back/forward buttons, does not have bookmarks, or an address bar. Remember this when designing your interface (you may been to provide in-app back/forward buttons, bookmarks, etc).
The only problems I ran into when developing this was that the API was unstable at the time (a few months ago) and that relative URLs for installation didn't work (https://bugzilla.mozilla.org/show_bug.cgi?id=745928) but the devs in the mozilla #webapps IRC channel were helpful.
The nice thing about the Marketplace is that you can specify which form factor you want your app available to - Desktop/Laptop, Phone, Tablet.
On the phone side, things are easy:
https://lh3.googleusercontent.com/-7uT3PPCyPD8/UP9RTN-392I/A...
https://lh3.googleusercontent.com/-GB4Vt6YVTKA/UP9RSDzDy8I/A...
https://lh5.googleusercontent.com/-dXnc0iuIBdM/UP9RRW_UILI/A...
https://lh6.googleusercontent.com/-NReWzbudobw/UP9RnJEiHpI/A...