Read this great introduction to Open Web Apps from MDN.
Open Web Apps are just websites with access to device features and follow Web specifications.
WebIDE
Firefox now ships with an IDE for Web Developers, called the WebIDE. You might have to download the Developer edition (affiliate link).
If you’re on the stable version, in about:config make sure devtools.webide.enabled is set to true
Press Shift+F8 to launch the webIDE.
If you don’t have a FirefoxOS phone, you’ll need to download a simulator addon.
Once you have everything ready, you can start building :D