I don't think this counts as official, but a Mozilla employee replied to a Reddit comment stating that it'll be open source too.
https://www.reddit.com/r/firefox/comments/5wio45/mozilla_acq...
[edit] also nnethercote is a Mozilla employee :)
https://www.reddit.com/r/firefox/comments/5wio45/mozilla_acq...
[edit] also nnethercote is a Mozilla employee :)