From MozillaZine Knowledge Base
Websites that are useful for Mozilla development. This knowledge base also has a few articles on extension development.
XML User Interface Language, widely used in Mozilla to specify user interfaces in a platform-independent way
Extensible Binding Language
An interpreted language, used for scripting UI in Mozilla
Document Object Model, an interface that allows scripts to dynamically access and update the content, structure and style of (HTML/XML) documents
XPCOM / XPConnect
Cascading Style Sheets
Resource Definition Framework
A bound publication of 49 or more pages that is not a magazine or periodical.
Various useful links