I was wondering where Fireflies were produced. It is my understanding that the majority of components (engine, driveline, etc.) came from the U.S. but were assembled in Britain. Is this the case?

The Firefly was a British conversion of the standard 75mm armed Sherman. The U.S. supplied complete Shermans to the British who in turn coverted them to Fireflies by adding the 17pdr, a turret bustle extension, new ammunition stowage, radios, etc. There was no actual assembly of the base Sherman tank in Britain.

Chris "toadman" Hughes
Toadman's Tank Pictures

IIRC the conversions were carried out at either Royal Ordnance Factories or depots. It's a while since I looked through Mark Hayward's Firefly Bible, but I think they might have been in Leeds and maybe Chilwell.

I'd point you towards Mark Hayward's website, but it seems to be down at the moment.