Of course I was still frustrated when they told the archers to "Fire!". "Loose" would have been more appropriate. Fire only came about after gunpowder/matchlock weapons were used!
Remember, though, the entire book/film is theoretically translated for us: in "reality", the characters spoke Westron rather than English, the hobbits had different names, etc. In the original, the term 'fire' would have been replaced with the Westron word for loosing an arrow; that was translated into 'fire' because 'fire' is the word familiar to modern English-speakers.
Or, wait, didn't Aragorn say that in Quenya? I can't recall whether it was in that or Westron.
Not that it matters: it's the same deal with both.