How to go back to an older version of Mozilla FireFox on Ubuntu Natty Narwhal.

I need to connect to my company's vpn using F5 vpn pluggin, it is a web-based vpn client that my company uses for remote connections to their network.

I upgraded my home computer to Ubuntu 11 Natty Narwhal and lost the ability to connect because FireFox 4 has no support for F5 plugin. I was forced then to revert or downgrade my version of FireFox to 3.6, but this was not as simple as I thought.

Step by step instructions: