Almost everything has now been moved over to (or replaced by) the XUL Content at the Mozilla Developer Center.

AAAAARGGGGHHHHHH !!!! Xulplanet was considerably faster to reach than devmo for a lot of people in Europe, including myself. Even if xulplanet was a bit outdated, it was also styled in a much more readable way than devmo. Devmo is nice and editable, xulplanet was practical and fast. Sigh...