|This mission is only available if you chose the Fire Crew approach for a The Bureau Raid heist. This mission is available to either Michael or Franklin.|
You should get a text after finishing up Architect's Plans, with Lester mentioning that you will need a Firetruck for the upcoming job. There are a few ways to go about getting one, such as going to the fire station or causing a fire somewhere. By far the easiest way in getting one is to use your phone to call Emergency Services and having them send a Firetruck.
Once it arrives, steal the Firetruck, but you're likely going to get a one-star wanted level. Lose the cops by hiding or driving around and when you're in the clear, bring the Firetruck back to the garment factory to finish the mission.