No SDK needed, it is all HTML5. If people talk about SDKs in that context, its often frameworks or maybe tools that can export to HTML5. But generally, you can use all the languages that work in a browser and use them without any framework or SDK to make an app.
The developer docs on Marketplace have this great intro on app development and testing: https://marketplace.firefox.com/developers/docs/quick_start
Happy Hacking!