A user could theoretically install an old, out-of-support version of Firefox (like version 51 or 52 ESR) alongside an old, out-of-support version of the Java 8 Runtime. They could then manually whitelist specific sites in the Java Control Panel and adjust Firefox’s plugin settings to "Ask to Activate." This is a catastrophic security practice, effectively drilling a hole in the hull of the ship to let in a small amount of water. It should only ever be done on an air-gapped, offline machine with no sensitive data.
If you encounter issues enabling Java in Firefox: enabling java firefox
Modern Firefox versions (released after 2017) do not natively support the Java plugin due to the removal of technology. If you need to run legacy Java applets in 2026, you must use specific workarounds, as the standard "Always Activate" options are no longer available in current releases. 1. Using a Modern Alternative (CheerpJ) A user could theoretically install an old, out-of-support
Enabling Java in Firefox requires navigating a significant shift in browser technology. Modern versions of Mozilla Firefox no longer support the standard Java plugin technology (NPAPI) that was used for decades. Consequently, simply installing the Java Runtime Environment (JRE) is insufficient to run Java content within the current release of the browser. To successfully enable Java, users must utilize the Firefox Extended Support Release (ESR). If you encounter issues enabling Java in Firefox:
However, this power came at a terrifying cost. The Java plugin became one of the most persistently exploited vectors for malware. Unlike the sandboxed, relatively limited environment of JavaScript, a Java applet had deep access to the user’s system. A single malicious applet on a compromised website could, in theory, escape its security manager and install ransomware, keyloggers, or botnet agents. The threat was not theoretical; year after year, major security reports listed Java as one of the riskiest pieces of software to keep enabled in a browser. The final death knell came from Oracle (Java’s owner) and the browser vendors themselves: in 2015, Oracle announced the end of the Java browser plugin’s support lifecycle. By 2017, Firefox version 52 (an Extended Support Release) became the last version to support the legacy NPAPI (Netscape Plugin Application Programming Interface) that Java required. Current versions of Firefox do not support NPAPI at all.
: While primarily for Chrome/Edge, developers can integrate CheerpJ into web environments to render Java content natively as WebAssembly.
If you have Java installed, but not the Java plugin for Firefox, follow these steps: