In an active shooter incident involving firearms, you should immediately lie on the ground.

In an active shooter situation, it is crucial to prioritize your safety and to respond appropriately. Immediately lying on the ground may not provide the best chance of avoiding danger. It may limit your ability to escape or find cover, both of which are essential actions in such a critical scenario.

Staying low to the ground can be beneficial if you are in a location where you cannot escape, as it can reduce your visibility to the shooter. However, the immediate instinct should be to assess your environment and decide whether it's safer to escape the area or hide behind something solid that can provide protection. Remaining aware of your surroundings and having a plan to get to safety is vital during such emergencies.

Thus, lying on the ground is not the best first action to take if there are opportunities to escape or find better cover.