First let me start with some links to documents which you better read up before venturing into mozilla world.
1. The whole of Mozilla is based on the technology called XPCOM which stands for Cross (X) Platform Common Object Model, which is pretty much similar to Microsoft COM. If you know COM you can draw similarities. You *must* know this. If not then these are the places to go:
- http://www.mozilla.org/projects/xpcom/ -- This is a page of links. The more you read, the better. (Obviously).
- http://www.mozilla.org/catalog/architecture/xpcom/ -- This has lesser links and makes more sense to read first. Some essentials are presented here. The IBM Developer works articles are good and are almost like hands on manual.
- http://www.mozilla.org/projects/xpcom/book/cxc/ -- This is linked from the above pages, but I am putting it here as it is quite important to read this one. The PDF can be handy.