If you build an app with Cordova you can easily get stuck in their APIs. If you build a Firefox OS web app you get stuck in Mozilla's APIs :) BUT Mozilla is making a strong effort at standardization and is working with Chrome and others to reach consensus. Cordova strays farther away from stable HTML5 APIs than Mozilla does so in Firefox OS you will definitely be building a more webby app. I.E. some HTML5 APIs work great on mobile, no reason to use a custom Cordova one in many cases.

However, if you need rich cross platform support for your app (like iOS) I'd still suggest going with Cordova. There is a Firefox OS backend! https://github.com/apache/cordova-firefoxos

You write a web app, throw it up anywhere, and it's an Open Web Application that can access anything the user lets it. There's no required "store", no need to enable sideloading, it's just a bunch of APIs you can access... on Firefox OS and Firefox for Android.

It's part of Mozilla's push in their portion of the mobile space to get away from the walled garden app ecosystem as much as possible.

We are working closely with the Cordova team to align our API's with those existing plug-ins. Meanwhile, this approach lets your app take advantage of Gecko's awesome WebGL and JS performance, plus goodies like WebRTC and Web Audio API.