My comments above are based on precedence when sites like Facebook added these clauses and people got all panicked thinking the company was going to start selling their content (rather than selling their souls /s). The mundane truth was that they needed the wording to make sure they were legally given the right to publish the content onto the web in the way they did. So people were assuming nefarious reasons when they were just legally protecting themselves.

Now, it does seem strange that Mozilla have suddenly added this when they haven't had it previously. Personally, I deem it highly unlikely that they are planning on monetizing our content in some way; whilst they have made some strange decisions sometimes I don't think they are completely stupid. Mozilla is in a precarious position right now, they are only managing to scape by on user trust and if that disappears they are finished. I'd like to think they are not foolish enough to do something that would catastrophically erode that trust, and selling user data to advertisers would kill them.

Having thought about it a bit more now, I have to wonder if they have dreamt up some other mad scheme, like Mozilla Cloud Storage, or something that would require such wording in the terms. Hopefully, it's just a wording update to protect themselves. I guess we will find out in due course.

[edit: fixed a typo.]