Does it actually say "firemens Lift" or is it just a standard passenger lift with a "Firemens Switch" behind a break glass cover?
If so the switch simply disables the landing call points, brings the lift to the access point and then enables the lift to be controlled only from the car.
Would you use it- Yes depending on the circumstances. Depends how much equipment you wuld otherwise have to carry up however many flights of stairs, where the fire is in relation to the lift, where the motor room is in relation to the fire, the fire in relation to the lift electrical supplies.
If its a flat going like the clappers in a tower block then you can be certain at least that they will be on seperate electrical supplies (landlords and tenants having seperate meters) and whatever the age of the building there should be plenty of compartmentation in place