Działa i tylko w takiej konfiguracji.
FirefoxProfile profile = new FirefoxProfile();
String pathe = "C:\\Ksiegowosctemp";
profile.SetPreference("browser.download.folderList", 2);
profile.SetPreference("browser.download.dir", pathe);
profile.SetPreference("browser.download.manager.alertOnEXEOpen", false);
profile.SetPreference("browser.helperApps.alwaysAsk.force", false);
profile.SetPreference("browser.helperApps.neverAsk.saveToDisk", "application/octet-stream");//,application/vnd.fdf,application/vnd.adobe.xfdf,application/pdf,application/x-pdf,application/acrobat,application/vnd.pdf,text/pdf,text/x-pdf,application/x-bzpdf,application/x-gzpdf,application/x-msexcel,application/excel,application/x-excel,application/excel,application/x-excel,application/excel,application/vnd.ms-excel,application/x-excel,application/x-msexcel");
profile.SetPreference("browser.download.folderList", 2);
profile.SetPreference("browser.download.dir", pathe);
profile.SetPreference("browser.download.manager.alertOnEXEOpen", false);
profile.SetPreference("browser.helperApps.neverAsk.saveToDisk", "application/msword, application/csv, application/ris, text/csv, image/png, application/pdf, text/html, text/plain, application/zip, application/x-zip, application/x-zip-compressed, application/download, application/octet-stream");
//profile.SetPreference("browser.download.manager.showWhenStarting", false);
profile.SetPreference("browser.download.manager.focusWhenStarting", false);
profile.SetPreference("browser.download.useDownloadDir", true);
profile.SetPreference("browser.helperApps.alwaysAsk.force", false);
profile.SetPreference("browser.download.manager.alertOnEXEOpen", false);
profile.SetPreference("browser.download.manager.closeWhenDone", true);
profile.SetPreference("browser.download.manager.showAlertOnComplete", false);
profile.SetPreference("browser.download.manager.useWindow", false);
profile.SetPreference("services.sync.prefs.sync.browser.download.manager.showWhenStarting", false);
profile.SetPreference("pdfjs.disabled", true);