As of September, 2018, Firefox no longer offers a version which supports NPAPI, the technology required to run Java applets. The Java Plugin for web browsers relies on the cross-platform plugin architecture NPAPI, which had been supported by all major web browsers for over a decade. The 64 bit version of Firefox has never supported NPAPI, and Firefox version 52ESR is the last release to support the technology. It is below the security baseline, and no longer supported.
Java applications are offered through web browsers as either a Java Webstart application (which do not interact with the browser once they are launched) or as a Java Applet (which might interact with the browser). This change does not affect Web Start applications, it only impacts Java Applets.
Developers and System administrators looking for alternative ways to support Firefox users should see this blog regarding Launching Web Start applications.