If you have GuardedID, right click on the "G" in the notification area. Then, choose "Check for Updates". Click on the popup that reads "GuardedID Cryptocolor is Available". It should state that it is asking you to install NoClickjack for Mozilla Firefox.

(Example using Windows 10)

A new window should open in Mozilla Firefox asking you to install NoClickjack. Click on "Click Here to Install", then choose "Add".