To install the Next-Generation Java plugin in firefox follow the instructions below which is a summary of this article. If you have been using the classic plugin, see the Notes for further information.

1. Check if you’re using the 32-bit firefox. Select the menu item Help -> About Mozilla Firefox. At the bottom of the window is a version string line that contains either “Linux i686″ (32-bit) or “Linux x86_64″ (64-bit). If you use the 64-bit version have a look at this blog post else continue.
2. Exit Firefox.
3. Change to ~/.mozilla/plugins or to another firefox plugins directory.
4. Remove all symlinks (or move them to another directory).
5. Create a symlink for the 32-bit plugin. Execute one of the following lines

   ln -s ${JRE_HOME}/lib/i386/libnpjp2.so
   ln -s ${JAVA_HOME}/jre/lib/i386/libnpjp2.so

   depending on what environment variable you have set. If you don’t know where your java is located find it out with which java or locate java

6. Start firefox.
7. Type about:plugins in your location bar to confirm that the java plugin is loaded.