`if !(navigator.apps && navigator.apps.install)` then we can't install a webapp for you. Let's add one of those little incompat messages so people know we know. Any strings should probably stay untranslated until fligtar/ragavan can take a look.

This is probably invalid if we start including the HTML5 shim.

This is almost ready but I need to know whether we want this message to show up all the time or whether we can suppress it once it has been seen.

All the time, assuming it's not an alert() or something. If it's just a note somewhere on the page, let's do it.

Done: https://github.com/mozilla/zamboni/commit/c36aebcf

Do we want this to show up on mobile?

Am I correct in thinking that `if !(navigator.apps && navigator.apps.install)` could only be true if the server that serves the Apps 'include.js' is down? Or does this also help us detect that the javascript interpreter is very, very old? How do we detect that javascript is totally turned off?

The only scenario in which navigator.mozApps isn't available is if the server providing include.js is down. This is, of course, assuming you are including the apps 'include.js' on all pages that needs to use the API: https://myapps.mozillalabs.com/jsapi/include.js (which backs off if it detects a native implementation provided by the add-on). Discussion on whether include.js should be hosted on AMO or elsewhere is in bug 695739.