Ford Fiesta misfiring at low revs

Depending on what engine is in car , one thing you could try is cleaning / replacing MAF (Mass Air Flow Sensor )

i had same problem on my 1997 fiesta misfire stalling jerky acceleration etc etc changed leads plugs coil idle valve ..drove me nuts for 3months ..was gonna changed fuel filter then cleaned lambda sensor (o2 sensor on manifold) and throttle sensor leads sprayed with wd40 seems to have cured problem power back no stalling for nearly a week ..might change the fuel filt as a matter of course ..but after other checks ..check the lambda and throttle sensors ..as my exhaust was sooty and is now normal grey colour ..the o2 sensor is often overlooked with the misfire problems ..good luck