By default Firefox has no Java plugin because of security issues.
One can install plugin by following next steps:
0. Exit Firefox browser if it is running
1. Make directory if it does not exist ->
2. Make a symbolic link for libnpjp2.so file which resides in JRE directory, e.g.:
sudo ln -s /usr/lib/jvm/jdk1.8.0_20/jre/lib/amd64/libnpjp2.so /usr/lib/mozilla/plugins/libnpjp2.so
Please note, that amd64 is an architecture of the OS you have installed, possibly it could be i386 in your case.
3. Start Firefox and type about:plugins in address box to check if browser able to see Java plugin.