Some applications on Firefox require Java for working. However, you can not install regularly on CentOS. I will show you how to install Java Plugin on CentOS for Firefox.
Step 1. Download the newest version of
Java (Linux RPM).
Step 2. Close the running Firefox browser
Step 3. Add permission for *.bin before installing
chmod a+x filename_rpm.bin
./filename_rpm.bin
Step 4. add java plugin into Firefox (it depends on your installing directory)
cd .mozilla/plugins
ln -s /usr/java/jre1.6.0_24/lib/i386/libnpjp2.so
Restart your Firefox!
Have fun!