This is about Pocket, which is what krn referred to as "a proprietary cloud synchronization service". Mozilla has been promising to make Pocket open source since it acquired it over 1.5 years ago, but haven't done it yet.
There is plenty of client source code available on github, the remaining parts are the server which understandably might pose some issues if it contains either patents or third party proprietary code.
Atleast part of the cloud service is open source (mozilla's side) and they are still planning to open source them. Until evidence of the contrary surfaces I do believe Mozilla when they say that.
