[paranoidrobot]

I can't speak for Mozilla, but I wouldn't be surprised to see GoDaddy and others used for non-core stuff.

The registrar for mozilla.org is MarkMonitor. I'd guess that most (if not all) of their big name/public facing domains are done through MarkMonitor.

Domains used for testing or marketing purposes might be done through GoDaddy and others. This is a fairly common pattern.