From all the articles I've read, the only people calling it "firecrackers" were passengers who heard "what sounded like a firecracker" or "firecrackers". Every news sources has been reporting for some time that he had a powder substance strapped to his leg and used a liquid, perhaps in a syringe, to cause the reaction.
MSNBC:
"Law enforcement officials said the burns indicated the explosive was strapped to his legs.
One U.S. intelligence official said the explosive device was a mix of powder and liquid. It failed when the passenger tried to detonate it."
BBC:
"Officials have described the device as a mixture of powder and liquid which failed to go off properly.
One terrorism expert said it looked as though a new way of concealing explosives on the body was involved."
