adjust to matched your jre install:
mkdir -p ~/.mozilla/plugins
cd ~/.mozilla/plugins
ln -s /usr/java
ln -s /usr/java/jre1.6.0_18/lib/amd64/libnpjp2.so
cd ~/.mozilla/plugins
ln -s /usr/java
ln -s /usr/java/jre1.6.0_18/lib/amd64/libnpjp2.so
and then restart firefox.