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.
There is no need to worry about who manufactured your breaker. Siemens and SquareD are two of the larger manufacturers. In fact there is likely to be no difference as long as both are labeled as type QP. Your panel has a label on it which will tell you what type of breaker is required. For instance mine tells me to use type C or BR. The physical connection to the busbar is what the type ensures. Poor contact with the busbar could result in fire, which is what a breaker is intended to prevent.
If you are not clear on how to do this you should hire a licensed electrician. You do not want to burn down your house.
Incidentally, you may see other brand names at your local hardware store, Westinghouse, Eaton, and GE (General Electric) to name a few. These are all manufacturers of breakers. Again, the important thing is that the breaker matches the type listed on the panel label and that the amperage or current draw (expressed in Amps, A, or a number on the lever of the breaker) matches the ampacity of the wire. Typical Romex (wire) in residential construction is capable of handling 15A of current and must be paired with a 15A breaker (or smaller - but 15 is the typical size - I don't even think there is a 10 or a 5 at most hardware stores). Again, doing this wrong may equal fire/loss of property/serious injury/death. If any of this hasn't frightened you into hiring an electrician, then read the part about death again. It's not a trivial thing and finding one that "fits" is a recipe for disaster.