MAF will have an arrow moulded in to the casing to indicate direction.
Don't know about yours, but the Barina had same size pipe in and out, so easy mistake to make.
Two more experiences for you.
My toyota had surging type problem when warm. Was a dud injector.
My Eunos 800M had a similar problem to your opening post in regards to the not revving above 2.5rpm.
Was a vacuum hose not fitted properly after a full service in which the supercharger had been taken off. Turned out to be common for engines with perished vacuum hoses.
With the age of your Fiesta it may be worth alook if there are any hoses on the fuel inlet systems.