We have removed direct access to Java from the DOM in Gecko. Specifically, we have removed support for the ‘java’ and ‘Packages’ DOM objects. These are not part of any web standards and we don’t believe they are good for the web.
Script can still interact with Java plugin instances via NPRuntime, authors simply have to instantiate a Java plugin instance to script against.
This change will be in Firefox 15. See Mozilla bug 748343 for more information.