In the context of the story, the galaxy has four zones: the unthinking
depths, the slow zone, the beyond, and the transcend. The region of the
beyond corresponds to the ocean, with the beyond/transcend boundary
being like the surface of the ocean and the slow zone/beyond boundary
being like the ocean floor. Much nautical terminology is used to
extend this analogy; for example, ships equipped to work near the slow
zone are called "bottom luggers" and ones which scoop into the slow
zone "dredges".
The "fire" then is the straumli perversion, the super intelligence
which is taking over a considerable portion of the transcend and the
upper beyond, threatening to penetrate even deeper into the beyond.
I believe the term "fire" is used here mostly to describe its destructive
properties, as well as emphasizing the unique ability of the perversion
to penetrate the beyond, which such attackers normally can't do, just
as fire normally can't burn within water. So the paradoxical imagery
of the title is matched by the unique events related in the story.
Hal