Ford Fiesta Owner's Manual page 181

The airbag SRS is designed to activate when the vehicle sustains lateral
deceleration sufficient to cause the sensors to close an electrical circuit
that initiates airbag inflation.
The fact that the airbags did not inflate in a collision does not mean that
something is wrong with the system. Rather, it means the forces were
not of the type sufficient to cause activation. Side airbags are designed
to inflate in side-impact collisions, not roll-over, rear-impact, frontal or
near-frontal collisions, unless the collision causes sufficient lateral
deceleration.
WARNING: Several air bag system components get hot after
inflation. Do not touch them after inflation.
WARNING: If the side
airbag has deployed, the
airbag will not function again.
The side airbag system
(including the seat) must be
inspected and serviced by an
authorized dealer. If the airbag
is not replaced, the unrepaired
area will increase the risk of injury
in a collision.
2012 Fiesta (fie)
Owners Guide, 3rd Printing
USA (fus)
Seating and Safety Restraints
181