I just noticed that all of the branches from my upstairs service box are run off of 15 or 20-amp type QP circuit breakers, except for one 15-amp line that is for the hard-wired smoke alarms- it uses a Square D type breaker. What is the difference between these two, and can they safely be mixed in a breaker box?
Some research says that a QP circuit breaker is the 1" or 2" wide breakers that protect a 110V or 220V circuit respectively, and is made by Siemens. Square D is another manufacturer of circuit breakers and service panels.
Ideally, you would want to have all your circuit breakers be manufactured by the same company that makes your service panel; the manufacturer will -- one hopes -- certify that their breakers will work correctly in their panels. Some manufacturers produce breakers with the same mechanical fit as others, in which case it would be up to that manufacturer to say if their breakers will work in a panel made by the other.
I'm assuming that the Square D breaker actually fits in the service panel, so to determine if it's OK, you'd have to do some research to see if Square D certifies their products to work with the panel, or if the panel's manufacturer does the same.
(Side note: per NEC2008, smoke detector circuits have to be protected with an AFCI if there's a detector in a bedroom. This might explain why that circuit is different.)
If the circuit breaker goes in without having to pound it in, I would say that it is probably fine. There are circuit breakers that are designed to go into more than one type of panel. I know that Square D makes breakers that work in multiple breaker panels, including QP panels.
As long as there isn't a significant voltage drop across the breaker, don't worry about it.
I would go along and test the voltage drop across all of the breakers while you are at it. That might give you an early warning of a faulty breaker.