Oil boiler fire valves are designed to stop the flow of oil if the boiler or house are on fire. However one of the most common reasons for ignition lock out with oil boilers is the fire valve shutting off without a fire.
As they get older they can get over sensitive and shut off the oil to the boiler at a much lower temperature than required.
The most common type of fire valves have a small black knob on their back that pops out when the valve is activated.
Resetting the valve is as easy as pushing that knob back in again.
If you find that the fire valve trips regularly I would recommend you get the boiler and valve checked by a qualified heating engineer.
It may be that the valve is faulty or it could be a sign of a fault in the boiler itself.