When My .jar is loaded - When is your jar loaded, do you deploy it after starting JBoss, is it in the deploy folder already, is it in the lib folder? What does your jar actually contain and what are you trying to do?
the Class Not Found exception - What is the exception and stack trace?
a particular class - Which class, one of yours - if so where is the class in relation to the app you are working with, one that is distributed with JBoss, one that is distributed with the JVM?
This class extends some other classes - Which classes, one of yours - if so where are the classes in relation to the app you are working with, classes that are distributed with JBoss, classes that are distributed with the JVM?