'Pouring out the vials upon the earth, which contained the plagues,' in Revelation 16:1, signifies influx of the reformed into the church, or into people who study and receive the doctrine of justification by faith alone.

(Odkazy: Apocalypse Explained 16; Apocalypse Revealed 676)

Fire, in the spiritual sense, can mean either love or hatred depending on the context, just as natural fire can be either comforting in keeping you warm, or scary in burning down your house. Our language reflects this, too -- we use concepts like a smoldering hatred or a burning love. So fire signifies a love, either a good love of the neighbor and to the Lord, or, in a bad sense, selfish love of oneself that, if unchecked by conscience, leads to hatred of anyone that opposes it.