About a year ago we wrote a post announcing plans to deprecate the Java browser plugin in JDK 9 due to browser vendors moving away from the standards-based NPAPI plugin support technology required to launch Java Applets.
Since then, the Oracle development team has published a JDK Enhancement Proposal (JEP 289: Deprecate the Applet API) with technical details about the planned deprecation step in JDK 9.
In addition, updated timelines for removal of standards-based plugin support from their browsers, eliminating the possibility to embed Java and other plugin based technologies, have been announced by the developers of Apple's Safari and Mozilla's Firefox. In accordance with their timeline, starting with Mozilla Firefox 52, due to release in March 2017, the 32-bit version of Mozilla Firefox will no longer provide the requisite APIs for standards based plugin support. The 64-bit version of Mozilla Firefox for Microsoft Windows launched without support for most NPAPI-based plugins including Java.
Mozilla offers an Extended Support Release (ESR) version of Firefox specifically for use by organizations who need extended support for mass deployments. Only the Mozilla Firefox 52 ESR 32-bit release will continue offering support for the standards-based plugin support technology required to launch Java Applets. Note that there is no way, on the server side, to distinguish between the general version and the ESR version of Firefox. Affected users may need further instructions on how to tell if they are using an ESR release (via "Help Menu -> About" and looking for the "ESR" identifier in Firefox 52) rather than developers implementing a server-side check for this.
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.
More background and information about different migration options can be found in this short whitepaper from Oracle.