At the time of writing this post, Oracle Forms 11g installations are configured by default to work with a specific JPI (Java Plug-In) version: 1.6.0_12.
You can verify this by checking your formsweb.cfg file:
grep jpi-version formsweb.cfg #jpi_mimetype=application/x-java-applet;jpi-version=1.6.0_12
Unfortunately, it seems Firefox is very particular when it comes to JPI versions, and it will only run with the version specified in your configuration. Internet Explorer and Chrome browsers however, seem to cope with other versions just fine. So when you trying loading an 11g form in Firefox, and it tries to load the Java applet, you get the following messages:
Additional plugins are required to display all the media on this page. Install Missing Plugins…
Now you could just install install JRE (Java Run-time Environment) 1.6 update 12, which contains the JPI, but a better solution would be to remove the restriction on a JPI version from your configuration like so:
export DOMAIN_HOME=/u01/app/oracle/middleware/user_projects/domains/ClassicDomain cd $DOMAIN_HOME/config/fmwconfig/servers/WLS_FORMS/applications/formsapp_11.1.2/config cp formsweb.cfg formsweb.cfg.DDMMYYYY vi formsweb.cfg
Comment out the jpi-version reference:
#jpi_mimetype=application/x-java-applet;jpi-version=1.6.0_12
…and add the following line:
jpi_mimetype=application/x-java-applet
Save the formsweb.cfg file (no restarts necessary).
Next time Firefox tries to load the Java applet for your form, it will the JPI you have installed 
Your solution will work, but if you want to retain the minimum JPI version check, just replace “jpi-version” with “version”. JRE 6 update 10 introduced the change to jpi-version and you should now use “version”. Oracle forms just hasn’t caught up with Oracle java yet.
Very handy to know, thanks Steve
Indeed
# Used for Sun JRE/JPI
jpi_mimetype=application/x-java-applet;jpi-version=1.4.2_15
Options for the above include the following:
In this example, the use of jpi-version will force the use of a specific or static version. This example would force the use of 1.4.2_15
jpi_mimetype=application/x-java-applet;jpi-version=1.4.2_15
In this example, the use of version is similar to using the Dynamic CLSID for IE. This example would permit any version 1.4 or newer to be used. It is important to not include the third digit (1.4.x) in order for this to work correctly.
jpi_mimetype=application/x-java-applet;version=1.4
Thank you very much, you are a genious, everything works fine :D,I love your tutorials.