When you say misfire do you mean it runs rough for the first couple of miles or does it keep missing intermittently?
I never had such a misfire on my 1.8 Zetec but I think I might have started by checking the coolant temperature sensor. It's on top of the thermostat housing at the rear of the head on the passenger side (RHD).
It very cramped around there and the wires are squashed under the heater pipe which could have chafed it and damaged the wires.
The reason I say that is because the ECU takes a coolant temperature signal and enriches the mixture until the engine gets up to running temperature.
If that has failed, it will default to the warm running mixture which will be too lean while the engine is cold. A bit like running an old carburettor engine without the choke.
If the wires look intact of course it could be a sensor failure but they are normally reliable.
Could be a host of other things as well but that's where I would start my diagnostics.
