Icedtea is the name of the open sourced version of the Java firefox plugin. It will be automatically installed with the OpenJDK PBI at www.pbidir.com
or you can install it from ports by going to /usr/ports/java/openjdk6 (or openjdk7)
/usr/ports/java/icedtea6-stubs. and running make install clean
If you use the Java PBIs, OpenJDK and Java JRE, you don't have to build the ports.
This should automatically load the plugin into your Firefox. Check for its presence by looking in tools-->addons-->plugins.
It should be listed under Icedtea-Web plugin. I hope this is helpful.