Mozilla expanding into social media feels like it will be walking a very delicate line regarding privacy. Things like Pocket have already been contentious enough.
They are putting a lot of emphasis on recommendation feeds and helping content publishers "build audiences", and of course there will ultimately be some form of (so far unspoken) monetisation. Mozilla are only going get so far with that until they start wanting user data, data which will be so temptingly convenient when it's tied to Mozilla accounts.
Chrome has already demonstrated the negative consequences of web browsers and web platforms becoming too intertwined. Maybe I'm just too cynical, but even with the best of intentions I'm not sure Mozilla can avoid the same fate here.