I did several Google searches very early this morning for Firex 120 1182. In case you're ever wondering about why the smoke detector is going off at 2:30 in the morning, it's because they're most likely to fail as it gets cooler.
The good news is, you can find a manual online eventually.
But, the short, 3 step version to solving the immediate problem is - if your detector is more than five years old, disconnect the power, take out the 9V backup battery, and if it it still continues chirping, put it outside :)
Step 3, proceed to Home Depot, buy a new version from Firex for $14, with a completely different model number, although essentially identical, and, hopefully, enjoy a good nights sleep.