A major problem with HMMWV vehicles damaged in combat, or accidents, is fire. The fuel tank is ruptured and a nasty fire ensues, often killing or injuring passengers trapped or knocked senseless (or unconscious). So the army installed a fire suppression system that consisted of two fire extinguishers that automatically activated when there was a fire inside the vehicle. But there was a problem. In many accidents, the electrical system was knocked out, and the fire suppression system with it.
The new version of the fire suppression system has a battery backup, and a third, manually activated, fire extinguisher, mainly for those who get to the vehicle to get people out. The rescuers can grab the third fire extinguisher and use it to keep the flames away from the passengers they are trying to get out. Conscious, but stuck, passengers can also use the third extinguisher.
Another solution to the fires is self-sealing fuel tanks. These were developed over 70 years ago, and used successfully in aircraft in World War II, and ever since. But the army was slow to adopt this technology for ground vehicles. This is slowly changing, with some of the larger armored trucks getting self-sealing fuel tanks. There is quite a lot of demand to install these tanks in hummers as well.