Imagine you're rushing through a burning building that you've never been in before, that's filled with nearly-opaque smoke. It would be pretty easy to get turned around, not knowing which way to go in order to get back out. That's a very real risk faced by firefighters on a regular basis, and it's why Capt. Jeff Dykes created the Northern Star.
The waterproof device adheres to the inside of a firefighter's mask, and incorporates a gyroscope, accelerometer and magnetometer. The latter is used to determine the cardinal direction in which the wearer is facing – really, it's an electronic compass. Remember, GPS doesn't work indoors.
Users power the device up simply by tapping on the mask twice. The Northern Star then lights up to show their heading, refreshing multiple times per second. Its lithium battery is claimed to outlast "even the longest fire" on a single charge, and it can be swapped out for a freshly-charged spare battery if need be.
The display will warn users when the battery is getting low, and also if any nearby magnetic fields are interfering with the magnetometer readings. Once the mask is removed, the device will automatically turn itself off after no movement is detected.
The Northern Star is currently the subject of a Kickstarter campaign, where a pledge of US$100 will get you one (which you could always give to your favorite firefighter) when and if it reaches production.
Sources: Northern Star Fire, Kickstarter