Except Android is no more or less closed than Firefox OS. Both open source, allow sideloading and have curated app stores.

Android itself isn't, but the ecosystem that they're building is. The market is controlled by Google and the apps will only run on Android phones. Not the OS is important, the ecosystem is. That's why Apple and Google and everyone are trying to keep everyone inside.

The Firefox OS app market is controlled by Mozilla and the apps will only run on Firefox OS (for now, at least).

Everyone is free to implement the Android app API, as Blackberry have, just like they are free to implement the Firefox OS phone APIs.

The difference is very small. Firefox is more likely to have portable apps, Android offers a much higher quality API for building apps.

There is an official marketplace offering from Mozilla: http://marketplace.firefox.com but others are free to implement their own and no one has to use the official one. The apps will run on both the OS and Firefox for Android, Firefox Desktop.

I tend to agree there are advantages to a packaged SDK but there are project initiatives such as Mortar to cover this base also.

  > The Firefox OS app market is controlled by Mozilla

One clarification here: Mozilla's marketplace code is entirely public (though confusingly named):

https://github.com/mozilla/zamboni

Any corporate or individual entity could take this code and set up their own entirely compatible platform.

While the Play backend isn't open-source, there are already at least two other competing app stores.

The differences really aren't that big.

Curiously, Google has produced another (enterprisey) system than disrupts the (more) closed Android system ... GWT. If you like the strong typing of Java, compiling it to Javascript seems "safe".