At the time of installation Forms services 11g is configured to work with a specific version of java run-time environment. You can find it in your FORMSWEB.CFG file. Following parameter in FORMSWEB.CFG file defines java plugin to be used,
# Parameter related to the version of the Java Plugin
jpi_mimetype=application/x-java-applet;jpi-version=1.6.0_12
Note that forms services is configured with JRE 1.6 update 12.
Each time Firefox tries to load java applet, it looks for java version 1.6.0_12. If your system has other version installed then you get following message,
Additional Plugins are required to display all the media on this page
To resolve this problem either install JRE 1.6 update 12 or edit you FORMSWEB.CFG to omit the java plugin as under
# Parameter related to the version of the Java Plugin
jpi_mimetype=application/x-java-applet
Now as FORMSWEB.CFG do not include any specific version java, Firefox will load java applet with any available java plugin.
Note that, there is no such issue with Internet Explorer.