How to install Firefox plugins like Java, Flash, PDF

Prerequisites: add universe and multiverse repositories.

How to install J2SE Runtime Environment (JRE) with Plug-in for Mozilla Firefox?
sudo apt-get install sun-j2re1.5
java -version

How to install Flash Player (Macromedia Flash) Plug-in for Mozilla Firefox?
sudo apt-get install flashplayer-mozilla

How to install PDF Reader (Adobe Reader) with Plug-in for Mozilla Firefox?
sudo apt-get install acroread
sudo apt-get install mozilla-acroread
sudo apt-get install acroread-plugins
Start Adobe Reader.

Finally you need to restart Mozilla Firefox.