The Mozdev FAQ explains the differences between mozilla.org and mozdev.org as follows:
Q: What is the difference between mozilla.org and mozdev.org?
A: mozilla.org provides a lot of the same development tools and resources as Mozdev does (including bugzilla, cvs hosting and newsgroups), but the focus of the two sites are on different things. The mozilla.org site is where the core Mozilla source code is developed. The projects hosted on Mozdev create applications and extensions that are based on top of that source code. For example, Firefox and Thunderbird are developed by mozilla.org and a large number of projects on Mozdev create extensions for these applications.