Hi,
you should bundle "jstl.jar" in your app. This file contains "fn.tld". If you have this JAR, but without the TLD, then maybe someone else did some cleanup and removed all TLD files from all JARs. In this case, you will have to undo this change :-(
Each JBoss version became more strict when parsing config files, and thus JBoss 6 performs more parsing than JBoss 5.
Best regards
Wolfgang