Support for the Java plugin and the Java applet in a browser window is dropped in Firefox 52 release, so you can no longer embed Java in a web page. Can't that website use modern HTML5 means in case you need it for a specific website? Websites need to use Java Web Start (i.e. send a special XML file as application/x-java-jnlp-file) to launch a Java application in an external standalone window and can no longer use an applet to open a Java application embedded in a web page. Websites that want to use Java need to adapt and make the change to launch a Java application this way.