To paraphrase: it seems they wanted a reading list feature but found it pointless to re-implement an existing solution with many desired features. (Work which was started but appears to have been scrapped.) This rationalized piggy-backing on Pocket. It gives Firefox a reading list for its users without Mozilla having to maintain it.
My personal problem with this has more to do with the anti-competitive nature of integrating services rather than Pocket's closed source.
[1] https://groups.google.com/forum/#!topic/firefox-dev/B3jJq_kU...