American flyers were convinced they were faced with a new German secret weapon. A popular comic strip of the era, Smokey Stover , may have contributed some to the name. Years later it became the name of a pretty good band . But that's another story. In some respects, though, they resembled ball lightning, artificially generated and under control from the ground.
Ball lightning is a fairly rare but well-documented phenomenon.
Artificial ball lighting can be generated with microwaves. You can even do it in your microwave oven. The Foo Fighters were far from the only advanced technology that the Nazis were developing when they were finally overrun. It's a great read and very well illustrated. No examination of the postwar UFO phenomenon is complete without it.