Sure is. The runtime for desktop apps is in desktop Firefox, but Mozilla has been focusing on mobile and hasn't really given enough PR to the desktop story. Expect to see more about desktop apps in the next year.
They can be installed in Firefox for Android too. For a while I ran an AOSP version of Android without the Google apps but with Firefox installed and using twitter, facebook, etc installed as 'Apps' from the Mozilla Marketplace. It's not quite a Firefox OS experience but not too bad.
Yessir! Developers can submit apps with the notion that they can run on Firefox Desktop, and it will work.
Anyone knows how to bypass OS X 's Gatekeeper to launch the app? Normal app is Ctrl-click to overwrite Gatekeeper blocking. But can't Ctrl -click on the "Launch" button on the page as it pop up context menu.
The app is also in your Applications directory, and you can Ctrl-click the icon in there.