The Firewalk Virus is used to complete the level 25 mission, Operation Firewalk.
Program the Firewalk virus by selecting [Turn on] next to your Personal Computer, and then selecting the Firewalk Virus from the list of viruses available for you to code.
In order to be able to code the Firewalk Virus, you need both to have completed the Expert Programming Course and be level 20.
The Firewalk Virus takes 120 Days to code
To see the other viruses that you can code, and some more information, go here.