The Perfect Catalyst Rated 5 out of 5 stars
This is simply awesome. If you know what you want to code and you know how to do it, this extension allows you to integrate the functionality into the UI without hassle. Thanks to toolbar customization, you can place your buttons wherever you like. Transparency works, you can just create a PNG from a screen shot and be done in no time. Creating, editing and deleting buttons using the context menu is intuitive and well integrated (e.g. if you have an error in your code, clicking the entry in the Error Console will take you directly to the code line that triggered the error). In SeaMonkey 2 it works in the browser and MailNews components alike.
Admitted, it might not be too beginner-friendly, but then I don't think it's the extension author's responsibility to make coding easy. If however you know what you're doing, this extension allows you to focus on actual development (creating something that does what you want) instead of having to worry about the UI integration. Without this extension you'd have to write your own, in addition to the actual code, just to have that.
Well done! :-)