XUL Booster is a third party Eclipse / WebTools build that offers some support for XUL Editing and awesome support for extension packaging.
XUL Booster makes creating and packaging an extension very easy. It supports exporting XPI files or registering a file system based project.
Create an extension
You'll need an extension ID; see Generating GUIDs.
Add an Overlay
You'll need the chrome address of the content you wish to overlay, ex. chrome://browser/content/browser.xul. Learn more at Creating_a_Mozilla_Extension:Finding_the_file_to_modify
To create an XUL:overlay, create a XUL fragment with a top level node that has an ID matching an existing element, ex. menupopup id="menu_ToolsPopup". Learn more at Creating_a_Mozilla_Extension:Adding_the_structure.
Visit XULBooster at SourceForge.net