Oracle has recently published a new post in the series “Moving to a Plugin-Free Web,” advising developers to find replacement solutions if they still have Java applets running in production. Firefox is going to stop supporting them soon.
Firefox, the last browser to support NPAPI plug-ins, is supposed to ditch them in early March when version 52 is planned to be released. Except for Flash, the general release of Firefox 32-bit will no longer support plug-ins, including Java applets. Support for Flash will be phased out at some point later. Mozilla builds a separate edition for enterprises called Extended Support Release (ESR) which comes with updates and fixes for 1 year, but these are not targeted to the general public. Firefox 52 ESR (32-bit) will continue to support plug-ins, being the only possibility to continue to run Java applets. Firefox 64-bit for Windows was released from the start without support for most plug-ins.
Oracle advises developers to quickly find a different solution if they are still using applets. JEP 289: Deprecate the Applet API is already “Completed.” As a result, several classes including Applet, JApplet and others, and the appletviewer utility are to be marked as “deprecated” when JDK 9 will be made generally available this summer, according to the current schedule. The respective classes are not to be removed now from the JDK but they will be removed later with another version. To migrate from applets run in the browser, Oracle recommends developers (PDF) to use one of the following solutions: Java Web Start, javapackager or JavaFX/WebView.
All major browser vendors complained for many years that plug-ins are a mechanism they do not like. Performance or security issues outside of their control made them decide to gradually remove support for such plug-ins, implementing native solutions for some of the gaps left behind such as using HTML5 for 2D drawings or video instead of using Flash.