Fuel pressure at the injectors is regulated. I don't know how many miles are on your Fiesta or to what degree it has been maintained. So, you might have already replaced the upstream O2 sensor.
If you have over 50 k on the odometer, I would replace that. It is basically the oxygen sniffer that determines the A/F ratio.
I have found that the lead oxygen sensors tend to slow down in their response as they age. I like to refer to them as "tired". They still function within the range of acceptable, but the response is not optimal. Prior to changing that is a good time to clean out the crap that accumulates in the airbox under the filter and get a spray can of throttle body cleaner to give that a good shot of Love.
Gummed up throttle bodies are also a big cause of the problem you describe.
If you really want to give it some TLC, an oil and filter change with a full synthetic oil such as Castrol titanium or Pennzoil platinum along with a fillup of high detergent premium fuel such as Mobil drive clean or Shell V-power wouldn't hurt it at all.