Refer here.

First you need to install JDK. In my case I install the Official JDK “jdk1.8.0_11(i386)” from Oracle.

Then run the following commands to let Firefox know it:

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mkdir ~/.mozilla/plugins/
ln -sf /home/user/java/jdk1.8.0_11/jre/lib/i386/libnpjp2.so ~/.mozilla/plugins/

Restart your Firefox, then it should work.