Several months ago we published Further Updates to Moving to a Plugin Free Web. Since then, Firefox 52 was released in March 2017) dropping support for NPAPI, impacting plugins for Java, Silverlight, and other similar NPAPI based plugins.
If you have problems accessing Java applications using Firefox, Oracle recommends using Internet Explorer (Windows) or Safari (Mac OS X) instead. But even then, developers and system administrators should be quickly migrating away from Applets,using Web Start applications as a stop gap.
There is, however, a final interim step that can be used for organizations relying on Firefox via the Firefox 52 ESR 32-bit release. Mozilla offers an Extended Support Release (ESR) version of Firefox specifically for use by organizations who need extended support for mass deployments. Only Mozilla Firefox 52 ESR 32-bit release will continue offering support for the standards-based plugin support technology required to launch Java Applets. To see if you are using an ESR release, check the Firefox menu item (Help -> About) and looking for the "ESR" identifier.
Mozilla maintains Firefox ESR Releases for approximately one year. Developers and users still relying on the Java plugin technology in the 32-bit Mozilla Firefox web browser should consider migrating to a different solution ASAP.