This is the “a-” preposition 1, forming a prepositional phrase “on fire” indicating being in a situation of fire.  It’s not to be confused with “a-” prefix 1, but could be parallel to “a-” prefix 3  with the sense of on or onto.  Bless.  I’m starting to get the hang of this.

• 14.015 and then another sprang afire

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“afire, adv. and adj.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.