That file, and many more, are located in a compressed file named embed.jar in the chrome folder. For my KM 1.5.2 version of the embed.jar file is 1,637 kb and has a modified date of Aug. 30, 2008. If you have 7zip then right click on embed.jar, choose 7zip, and open the archive. Drill down to kmprefs and then content. Pref.xul should be there with a size of 11 kb.
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