The default download location of the local Firefox browser is /tmp/.
Open a Local Terminal and log in as root
Copy the downloaded .jar file from the /tmp/ directory to /wfs/simplecalc.jar: cp /tmp/ /wfs/simplecalc.jar It is important to copy the file to /wfs because otherwise the file would be flushed with a reboot.
Open IGEL Setup and create a new custom application: System > Firmware Customization > Custom Application
Set Command to java -jar /wfs/simplecalc.jar in Settings
Click the newly created icon on the desktop to run the custom application.
To distribute this application to several thin clients please use the file transfer option in IGEL UMS and set up a profile with the custom application configuration.