Warzone Firebrand blueprint can be found after players discover a certain Easter egg. It's hidden in the new subway system which allows players to fast travel around the map. If you're going to complete this Easter egg puzzle, you'll need to go to the right subway station.
First head to the City Hall building in the downtown area and head to the small bank desk. You'll find a keypad. Input 2179 and watch as the room fills with gas. Use the computer near the keypad and memorize the numbers and symbols on the screen. Go back the way you came into the main room and find the paintings connected to the numbers from the PC.
Warzone Firebrand Blueprint Hidden Behind New Subway Easter Egg
You'll have a new code to decipher and match with the symbols from the PC screen. Find the nearest subway station and ride it to the airport station. Go into the station's lobby and find the PC in the side room and interact with it Wait for the A.E. Train Arriving Soon message to appear on the screen. Once it does, hop on the train and you'll be taken to the bunker.
The bunker will have a ton of good loot including the new Foresight killstreak, a Juggernaut suit, and the Firebrand weapon blueprint for the Bruen MK9.