Will a SimpliSafe security system which include smoke detectors qualify under fire panel exception for the NEC's AFCI requirements?
Since you have separate appliances, it's a NFPA 72 residential (combination) system, and thus 760.41 and its associated exceptions apply

You are correct that NEC 760.41(B) waives GFCI and AFCI requirements for power sources to fire alarm systems; in fact, it outright prohibits the use of GFCIs and AFCIs on fire alarm system power sources:


  (B) Branch Circuit. The branch circuit supplying the fire
  alarm equipment(s) shall supply no other loads. The location
  of the branch-circuit overcurrent protective device shall be
  permanently identified at the fire alarm control unit. The
  circuit disconnecting means shall have red identification, shall
  be accessible only to qualified personnel, and shall be identified as “FIRE ALARM CIRCUIT.” The red identification shall
  not damage the overcurrent protective devices or obscure the
  manufacturer's markings. This branch circuit shall not be
  supplied through ground-fault circuit interrupters or arc-fault
  circuit-interrupters.


However, in order to apply this, we need to know what sort of fire alarm system we have, and in order to do that, we need to turn to NFPA 72, which is the governing code in the USA for fire alarms of all sorts (from single station smoke detectors to massive systems that link together multiple high-rise buildings).

In the SimpliSafe's case, the system, in order to perform its fire alarming function, requires:


At least one compatible smoke detector (initiating device)
The base station/central hub (fire alarm control unit)
And some sort of sounder (notification appliance)


Since we have separate parts involved, we know it doesn't fit the definition of a single station alarm as those are defined as all-in-one units by NFPA 72:


  3.3.245 Single Station Alarm. A detector comprising an assembly that incorporates a sensor, control components, and
  an alarm notification appliance in one unit operated from a
  power source either located in the unit or obtained at the
  point of installation.


And, since we're not dealing with single station alarms, we know it's not a multiple station alarm either:


  3.3.149 Multiple Station Alarm. A single station alarm capable
  of being interconnected to one or more additional alarms so
  that the actuation of one causes the appropriate alarm signal
  to operate in all interconnected alarms.


From that point, we can deduce that it is a fire alarm system:


  3.3.95 Fire Alarm System. A system or portion of a combination system that consists of components and circuits arranged
  to monitor and annunciate the status of fire alarm or supervisory signal-initiating devices and to initiate the appropriate
  response to those signals.


and from the fact it's in a dwelling unit (house), we can then determine that it's a household fire alarm system:


  3.3.95.2 Household Fire Alarm System. A system of devices
  that uses a fire alarm control unit to produce an alarm
  signal in the household for the purpose of notifying the
  occupants of the presence of a fire so that they will evacuate
  the premises.


that also happens to be a combination system:


  3.3.95.1* Combination System. A fire alarm system in which
  components are used, in whole or in part, in common with
  a non-fire signaling system.


as it can perform the other functions expected of a household alarm system, such as burglary alarming.

As a result of this, since we have a fire alarm system using a separate fire alarm control unit:


  3.3.92* Fire Alarm Control Unit. (FACU) A component of the
  fire alarm system, provided with primary and secondary power
  sources, which receives signals from initiating devices or other
  fire alarm control units, and processes these signals to determine part or all of the required fire alarm system output function(s).


, we then treat the power source to the fire alarm control unit as a power source to a NPLFA circuit, which invokes NEC 760.41.  (Granted, NFPA 72 itself is evolving in this regard, and as of 2016, best I can tell, is not as stringent as the NEC is here.)