在XUL中设置java类（jar文件）的类路径，通过javascript访问

I am implementing firefox extension using XUL. I have set of java classes that is zipped as jar which needs to be accessed through javascript. If I copy my jar into jre's lib/ext then javascript will be able to invoke my java classes. If I remove from ext and package this jar in xpi, Javascript is unable to find the classpath for my jar file. Please let me know how to set the classpath for my java classes(jar file) in XUL environment at the earliest.I am implementing firefox extension using XUL.