I haven’t found XML Schema definition file for Openfire plugins configuration (plugin.xml), so I’ve created one and uploaded it as an attachment to a document describing how to use it under Eclipse. Here is the document: Creating and validating plugin.xml files with Eclipse using XML Schema.
I’ve checked against it all plugin.xml files from trunk and they are validating correctly. Using XML Schema files in Eclipse or oder IDEs will allow validation and content assist. I hope someone will find it useful.