For a long time, Mozilla has required people gaining commit access to our core repos to sign a Committer’s Agreement. This is not a copyright assignment or a transfer of rights; it’s basically a commitment to good behaviour, and to making sure code which gets into the tree is allowed to be there and is correctly licensed.
However, the logistics of printing it out, signing it, scanning/photographing it back in etc. were always a barrier to participation. In consultation with our legal team, we have decided that people simply assenting to the document is just as good so, as of now, people are no longer required to go through the process of signing it.
However, all people with commit access to any Mozilla repository are still expected to abide by it :-) We may be adding CONTRIBUTING files referencing the document to our Github repos to make this point more clear.