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.