The FireGoose is a Mozilla Firefox extension that connects Firefox to the Gaggle.
XUL, javascript, XPCOM, Java, and RMI.
We use code from the java-firefox-extension to allow us to load our own Java code in an extension. Official support for that is coming in future versions of the Mozilla platform in the form of JavaXPCOM.
We’re also considering trying to implement a Goose purely in Javascript using sockets and XML or SOAP. (see @mozilla.org/network/socket-transport-service.)