I ran a search in Google Books for the terms "firewall" and "fire wall." There are not a lot of hits overall. Prior to about 1910, the word appears only in the context of buildings or ships. Around 1910 it starts to be used in the context of airplanes. The earliest I found it used in the context of cars was 1931, in a book called "Perpetual Trouble Shooter's Manual" which was actually about troubleshooting radios. It also appears in a radio service manual from 1936.
Since these usages are from outside the auto industry, it's certainly possible that the authors were creatively applying a term used with aircraft to what they considered an analogous part in cars, rather than choosing a term with an established definition in the automotive context.
When I looked up the use of "dash" in this context, I didn't find any uses that made it clear that the term applied to the entire panel we think of as the firewall. It was mostly used the way we use it today, to describe the upper part of the firewall that has instruments attached to it. So possibly there's a third term that's a more direct substitute for firewall that was used previously.
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