Chat Core

Chat Core is the shared code for instant messaging that is shared by Instantbird and Thunderbird. It provides a number of functions and capabilities, including:

Chat Core Protocols: The Chat Core code used by Instantbird and Thunderbird has some abstractions to deal with the differences between protocols (e.g. IRC vs. XMPP).
Contacts: Contacts are at the heart of instant messaging, and thus the Chat Core has a way to abstract to a "person" (represented by an imIContact instance), which might connect to multiple networks, etc.
Keyboard shortcuts: This is a list of keyboard shortcuts that are available in Instantbird/Thunderbird with brief descriptions of what they do.
Message Styles: Chat Core uses a message style system based on HTML, JS and CSS that is very similar to the one created for Adium. If you plan to create a message style for Instantbird or Thunderbird, reading the Adium documentation on the topic may be helpful -- see this tutorial and this reference sheet.

Notifications: This is a page for documenting the notifications in Instantbird and Thunderbird. This is likely out of date. Notifications are grouped by interface you need to attach the observer to.
Smiley Themes: A smiley theme for Chat Core (used by Instantbird and Thunderbird) is very easy to create, it contains a bunch of image files, a theme.js file and the necessary packaging around it to make it an extension.