2000 Ford Fiesta vibration on break pedle at low speed

If it had the same problem prior to fitting new tyres, check the rear brake drums(probably oval)and require skimming. If only after fitting tyres take it back to the fitment centre and let them have a look. A good piece of advice; After driving in stop go traffic, don't pull up the handbrake tight as the drums are very hot and when they cool they will shrink onto the brake shoes thus rendering the drums oval.

i would suspect wot has happened as they removed the front wheels to fit tyres using an impact wrench is dislodges rust behind the brake disc thus making it not run true, due to the discs have no retaining screws/bolt . req discs of and clean or renew

i would go back to the place where you had the tyres and just have them give the car a once over just to be safe and check the wheel nuts are tight just before doing any further journeys in the car and have the wheels balanced up again as this can lead to vibration at certain speeds also like mastertech has said have the discs and pads checked if not replaced with it being a fiesta i would have said they will cost about £45 plus vat but have them checked first