The lyrics contain apocalyptic, biblical themes. Bono has always worked a lot of religious imagery into his lyrics, but he's rarely this dire, singing about the "sun burning black" and "stars falling down."
"Fire" was released as a single three months before U2's second album, October, came out. It was the first U2 song to chart, going to #35 in the UK. It was produced by Steve Lillywhite, who helmed the first three U2 albums.
U2 recorded this in the Bahamas in 1981 while on a break from their US tour. It was the only track on October not recorded in Dublin.
In their autobiography U2 by U2, the band calls this one of their least favorite songs, with the Edge noting, "The problem was it was high in potential but actually not very high on content."
A distorted guitar segment of this appears at the end of some of the vinyl copies of Boy.
U2 performed this on their first appearance on British TV show Top Of The Pops.