Apart from being an elegant debugger, FireBug offers the developer hooks that can be used from JavaScript, offering logging, assertions, performance measuring, and command line functions for inspection or traversing a document.
A collection of gotchas and guidance in Greasemonkey scripting. Positioned as an addition to Mark Pilgrim's dive into Greasemonkey
Mark Pilgrim dissects the security issues of the 0.3 Greasemonkey, and describes how the new architecture of Greasemonkey works, pointing out pitfalls and how to address them.