Hi jilarde,

The Marketplace on Firefox OS has been shut down since 30 march 2018.

https://wiki.mozilla.org/Marketplace

There are some archive and dumps of the apps mentioned in https://github.com/mossroy/marketplace-backup/issues/1

However it is not like side loading a .apk on Android as you will have to look for how to load apps onto your device, if it is still possible.

Note to others, I am aware this thread is in desktop Firefox section as there is no option to move it to Firefox OS section anymore so no need to report this to be moved.