Chat Core is the code for instant messaging that is used by Thunderbird. It provides a number of functions and capabilities, including:
The Chat Core code used by Thunderbird has some abstractions to deal with the differences between protocols (e.g. IRC vs. XMPP).
This is a list of the available keyboard shortcuts with brief descriptions of what they do.
This is a page for documenting the notifications in Thunderbird. This is likely out of date. Notifications are grouped by interface you need to attach the observer to.