By default there is no Java persay only Javascripts is my understanding that Firefox uses. It's up to the user to decide if they want to install Java RE if they need it or not.

Firefox has not supported any NPAPI Plugins since Firefox 52.0 Release (March 2017) with the exception of Flash Player plugin from Adobe for a while. No modern web browser supports Java plugin now.

Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work? https://support.mozilla.org/en-US/kb/npapi-plugins

Note the Java Plugin from Oracle and JavaScript in Firefox are two separate things. http://kb.mozillazine.org/JavaScript_is_not_Java

You posted with a Firefox Firefox 53.0.3 user agent. Any specific reason for that specific version ?

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.