How to install the Java plugin for Firefox?

If you are using a version after 52 (not ESR), which disabled NPAPI, you are out of luck. If you are using one prior, which you absolutely wouldn't want to, you can install the OpenJDK JRE, which would allow you to execute Java binaries locally using the java binary. This however, isn't enough to execute Java web applets on your browser, which can be accomplished by installing the Icedtea plugin:

sudo apt-get install icedtea-plugin

Then go to about:addons, in the plugin section, check if the plugin is enabled. Mozilla disables insecure plugins by default, that means that Java is disable even after installation due security concerns. You need to activate it before using it.

Installing the IcedTea Java plugin:

To install the IcedTea Java plug-in, open a terminal and type:

sudo apt-get install icedtea-7-plugin

Install the Firefox plug-in - Oracle

In a terminal:

mkdir ~/.mozilla/plugins

Remove the IcedTea plugin, if it has been installed.

sudo apt-get remove icedtea6-plugin

Remove a former version of the Java plugin (may or may not be present)

rm ~/.mozilla/plugins/libnpjp2.so

Now you can install the plugin, by creating a symbolic link (you tell Firefox where the plugin is located). For 32-bit Java, use

ln -s /usr/lib/jvm/jre1.7.0/lib/i386/libnpjp2.so ~/.mozilla/plugins/

For 64-bit Java use

ln -s /usr/lib/jvm/jre1.7.0/lib/amd64/libnpjp2.so ~/.mozilla/plugins/

Confirm that the JRE has been successful by using the official Oracle website.

Source: Java installation

Beginning in Firefox version 52, support for NPAPI plugins (java applets) in Firefox has ended, due to performance and security issues, the way that I manage to run java applets is to install the Extended Support Release (ESR) of firefox.

download here: https://www.mozilla.org/en-US/firefox/organizations/all/

more details here: https://support.mozilla.org/en-US/kb/npapi-plugins