Issue exporting har file for a single (non-html) request

[nmcconom]

I have a Java/Selenium program that exports that har files using firebug/netexport combination. It works ok for real sites but I also try it with some files of various sizes - they typically just contain random characters like below example:

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

When I check the exported har though it only contains:

{
"log": {
"version": "1.1",
"creator": {
"name": "Firebug",
"version": "2.0"
},
"browser": {
"name": "Firefox",
"version": "30.0"
},
"pages": [],
"entries": []
}
}

Whereas if I request it manually then it exports OK.

I set the following preferences for the java tool:

// Set default Firefox preferences
profOpt.setPreference("app.update.enabled", false);
String domain = "extensions.firebug.";

                // Set default Firebug preferences
                profOpt.setPreference(domain + "currentVersion", "2.0.2");
                profOpt.setPreference(domain + "allPagesActivation", "on");
                profOpt.setPreference(domain + "defaultPanelName", "net");
                profOpt.setPreference(domain + "net.enableSites", true);

                // Set default NetExport preferences
                profOpt.setPreference(domain + "netexport.alwaysEnableAutoExport", true);
                profOpt.setPreference(domain + "netexport.showPreview", false);
                profOpt.setPreference(domain + "netexport.defaultLogDir", outputDir);
                profOpt.setPreference(domain + "netexport.autoExportToFile", true);
                profOpt.setPreference(domain + "net.defaultPersist", false);
                profOpt.setPreference(domain + "console.defaultPersist", false);
                profOpt.setPreference(domain + "netexport.includeResponseBodies", false);
                profOpt.setPreference(domain + "netexport.pageLoadedTimeout", 0);
                profOpt.setPreference("extensions.firebug.netexport.secretToken", "abcd");
                profOpt.setPreference(domain + "net.logLimit", 4000);
                // Prevent favicon requests which can screw up timings
                profOpt.setPreference("browser.chrome.favicons", false);
                profOpt.setPreference("browser.chrome.site_icons", false);
                // Disable IPv6 DNS lookups
                profOpt.setPreference("network.dns.disableIPv6", true);

Any idea what might be happening?