The HTML5 implementation of Jmol, JSmol, works just fine in Firefox. Until sometime in 2018, there was a special version of Firefox called Extended Support Release (ESR) that supported Java however, later ESR versions no longer support Java. That change happened in March 2017 and had been announced well in advance. The Mozilla Firefox browser does not support Java (including the Java-dependent version of Jmol) since version 52 of the 32-bit browser the 64-bit Firefox has never supported Java.

The HTML5 implementation of Jmol, JSmol, works just fine in Chrome. This change happened in late 2015 and had been announced well in advance. The Google Chrome browser does not support Java (including the Java-dependent version of Jmol). Official browser compatibility documentation for Java. Java Applets do not work in Chrome, Firefox, Edge, or Opera 7.1.1.2 Internet Explorer: Enabling Java.

7.1.1.1 Accessing Internet Explorer in Windows 10.1 Java Applets do not work in Chrome, Firefox, Edge, or Opera.

Mozilla firefox esr java