Thank you @Timo_Kuisma1. But like I mentioned in my post: I did already try to apply your solution via the Katalon project settings > Desired Capabilities for Chrome (headless). See my supplied screenshot above.
I can see below settings in my settings\internal com.kms.katalon.core.webui.chrome (headless)
{"HEADLESS_DRIVER":{"prefs":{"download.prompt_for_download":false,"download.directory_upgrade":true,"safebrowsing.enabled":false,"safebrowsing.disable_download_protection":true,"download_dir":"C:\\KatalonDownloads","profile.default_content_settings.popups":"0","download.default_directory":"C:\\KatalonDownloads","download":{"default_directory":"C:\\KatalonDownloads"}},"excludeSwitches":["enable-automation"],"args":["--disable-infobars"]}}
I tried also âprofile.default_content_settings.popupsâ as a number which is then set to 0.0 by Katalon. Doesnât seem to download it. The click action was successful, I left enough time to allow to finish the download,⊠Works on Chrome, not on headless Chrome. 
@Russ_Thomas : Thanks for your reply. I did however already read your handy topic, but again, it works fine on normal Chrome (with the below settings), but itâs the Chrome Headless where it doesnât.
{"CHROME_DRIVER":{"excludeSwitches":["enable-automation"],"useAutomationExtension":false,"prefs":{"download.default_directory":"C:\\KatalonDownloads"}}}
As a workaround, I tried to get it working for Firefox. But there I get annoyed by the save/open dialog which I canât seem to get through. I supplied following desired capabilities (like suggested in topic Automatic download on firefox and here https://stackoverflow.com/questions/36309314/set-firefox-profile-to-download-files-automatically-using-selenium-and-java ), but my profile does NOT seem to be applied when running in Katalon. Canât figure out why⊠These are my firefox desired capabilites settings:
{"FIREFOX_DRIVER":{"firefox_binary":"C:\\Users\\ABC\\AppData\\Local\\Mozilla Firefox\\firefox.exe","firefox_profile":{"browser.download.folderList":2.0,"browser.helperApps.alwaysAsk.force":false,"browser.download.manager.showWhenStarting":false,"browser.download.dir":"C:\\KatalonDownloads","browser.download.downloadDir":"C:\\KatalonDownloads","browser.download.defaultFolder":"C:\\KatalonDownloads","browser.helperApps.neverAsk.saveToDisk":"application/pdf;charset\u003dUTF-8","security.insecure_field_warning.contextual.enabled":false,"browser.helperApps.neverAsk.openFile":"application/pdf;charset\u003dUTF-8","marionette":true,"pdfjs.disabled":true}}}
When I debug and click manually on the download button in the firefox popup I notice it doesnât even get downloaded in my expected download.dir C:\KatalonDownloads. So I suspect my profile to not be applied for Firefox. Also here, no idea why.
This is the log message in the Console:
2020-05-20 14:40:48.931 INFO c.k.katalon.core.main.TestCaseExecutor - START Test Cases/Projects/XYZ/Test/Functional/downloadFullRequest
2020-05-20 14:40:49.803 INFO c.k.katalon.core.main.TestCaseExecutor - (Default) downloadRequestType = fullRequest
2020-05-20 14:40:49.952 INFO com.kms.katalon.core.util.KeywordUtil - testCaseContext.getTextCaseId()=downloadFullRequest
2020-05-20 14:40:51.253 INFO c.k.k.core.webui.driver.DriverFactory - Starting âFirefoxâ driver
mei 20, 2020 2:40:51 PM org.openqa.selenium.remote.DesiredCapabilities firefox
INFO: Using new FirefoxOptions()
is preferred to DesiredCapabilities.firefox()
2020-05-20 14:40:51.591 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set preference: [âfirefox_binaryâ, âC:\Users\ABC\AppData\Local\Mozilla Firefox\firefox.exeâ]
2020-05-20 14:40:51.592 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.download.folderListâ, â2â]
2020-05-20 14:40:51.594 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.helperApps.alwaysAsk.forceâ, âfalseâ]
2020-05-20 14:40:51.595 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.download.manager.showWhenStartingâ, âfalseâ]
2020-05-20 14:40:51.595 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.download.dirâ, âC:\KatalonDownloadsâ]
2020-05-20 14:40:51.618 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.download.downloadDirâ, âC:\KatalonDownloadsâ]
2020-05-20 14:40:51.621 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.download.defaultFolderâ, âC:\KatalonDownloadsâ]
2020-05-20 14:40:51.623 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.helperApps.neverAsk.saveToDiskâ, âapplication/pdf;charset=UTF-8â]
2020-05-20 14:40:51.625 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âsecurity.insecure_field_warning.contextual.enabledâ, âfalseâ]
2020-05-20 14:40:51.626 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âbrowser.helperApps.neverAsk.openFileâ, âapplication/pdf;charset=UTF-8â]
2020-05-20 14:40:51.627 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âmarionetteâ, âtrueâ]
2020-05-20 14:40:51.628 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [âpdfjs.disabledâ, âtrueâ]
2020-05-20 14:40:51.630 INFO c.k.k.core.webui.driver.DriverFactory - Action delay is set to 0 milliseconds
1589978462305 mozrunner::runner INFO Running command: âC:/Users/ABC/AppData/Local/Mozilla Firefox/firefox.exeâ â-marionetteâ â-foregroundâ â-no-remoteâ â-profileâ âC:\Users\ABC~1\AppData\Local\Temp\rust_mozprofile.KwcXF9b8qprjâ
1589978463412 addons.webextension. WARN Loading extension ânullâ: Reading manifest: Warning processing permissions: Error processing permissions.12: Value âdebuggerâ must either: must either [must either [be one of [âclipboardReadâ, âclipboardWriteâ, âgeolocationâ, âidleâ, ânotificationsâ], be one of [âbookmarksâ], be one of [âfindâ], be one of [âhistoryâ], be one of [âmenus.overrideContextâ], be one of [âsearchâ], be one of [âtopSitesâ], be one of [âactiveTabâ, âtabsâ, âtabHideâ], be one of [âbrowserSettingsâ], be one of [âcookiesâ], be one of [âdownloadsâ, âdownloads.openâ], be one of [âprivacyâ], be one of [âwebNavigationâ], or be one of [âwebRequestâ, âwebRequestBlockingâ]], be one of [âalarmsâ, âmozillaAddonsâ, âstorageâ, âunlimitedStorageâ], be one of [âbrowsingDataâ], be one of [âcaptivePortalâ], be one of [âdevtoolsâ], be one of [âidentityâ], be one of [âmenusâ, âcontextMenusâ], be one of [ânormandyAddonStudyâ], be one of [âpkcs11â], be one of [âsessionsâ], be one of [âurlbarâ], be one of [âgeckoProfilerâ], be one of [âcontextualIdentitiesâ], be one of [âdnsâ], be one of [âactivityLogâ], be one of [âmanagementâ], be one of [ânetworkStatusâ], be one of [âproxyâ], be one of [ânativeMessagingâ], be one of [âtelemetryâ], be one of [âthemeâ], or match the pattern /^experiments(.\w+)+$/], or must either [be one of ["<all_urls>"], must either [match the pattern /^(https?|wss?|file|ftp|*)://(*|*.[^/]+|[^/]+)/.$/, or match the pattern /^file:///.$/], or match the pattern /^resource://(*|*.[^/]+|[^/]+)/.$|^about:/]
1589978463413 addons.webextension. WARN Loading extension ânullâ: Reading manifest: Warning processing offline_enabled: An unexpected property was found in the WebExtension manifest.
1589978463534 addons.webextension.doh-rollout@mozilla.org WARN Loading extension âdoh-rollout@mozilla.orgâ: Reading manifest: Invalid extension permission: networkStatus
1589978463582 addons.webextension.screenshots@mozilla.org WARN Loading extension âscreenshots@mozilla.orgâ: Reading manifest: Invalid extension permission: mozillaAddons
1589978463582 addons.webextension.screenshots@mozilla.org WARN Loading extension âscreenshots@mozilla.orgâ: Reading manifest: Invalid extension permission: telemetry
1589978463583 addons.webextension.screenshots@mozilla.org WARN Loading extension âscreenshots@mozilla.orgâ: Reading manifest: Invalid extension permission: resource://pdf.js/
1589978463583 addons.webextension.screenshots@mozilla.org WARN Loading extension âscreenshots@mozilla.orgâ: Reading manifest: Invalid extension permission: about:reader
1589978465021 addons.webextension.{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a} WARN Loading extension â{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a}â: Reading manifest: Warning processing permissions: Error processing permissions.12: Value âdebuggerâ must either: must either [must either [be one of [âclipboardReadâ, âclipboardWriteâ, âgeolocationâ, âidleâ, ânotificationsâ], be one of [âbookmarksâ], be one of [âfindâ], be one of [âhistoryâ], be one of [âmenus.overrideContextâ], be one of [âsearchâ], be one of [âtopSitesâ], be one of [âactiveTabâ, âtabsâ, âtabHideâ], be one of [âbrowserSettingsâ], be one of [âcookiesâ], be one of [âdownloadsâ, âdownloads.openâ], be one of [âprivacyâ], be one of [âwebNavigationâ], or be one of [âwebRequestâ, âwebRequestBlockingâ]], be one of [âalarmsâ, âmozillaAddonsâ, âstorageâ, âunlimitedStorageâ], be one of [âbrowsingDataâ], be one of [âcaptivePortalâ], be one of [âdevtoolsâ], be one of [âidentityâ], be one of [âmenusâ, âcontextMenusâ], be one of [ânormandyAddonStudyâ], be one of [âpkcs11â], be one of [âsessionsâ], be one of [âurlbarâ], be one of [âgeckoProfilerâ], be one of [âcontextualIdentitiesâ], be one of [âdnsâ], be one of [âactivityLogâ], be one of [âmanagementâ], be one of [ânetworkStatusâ], be one of [âproxyâ], be one of [ânativeMessagingâ], be one of [âtelemetryâ], be one of [âthemeâ], or match the pattern /^experiments(.\w+)+$/], or must either [be one of ["<all_urls>"], must either [match the pattern /^(https?|wss?|file|ftp|*)://(*|*.[^/]+|[^/]+)/.$/, or match the pattern /^file:///.$/], or match the pattern /^resource://(*|*.[^/]+|[^/]+)/.$|^about:/]
1589978465021 addons.webextension.{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a} WARN Loading extension â{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a}â: Reading manifest: Warning processing offline_enabled: An unexpected property was found in the WebExtension manifest.
JavaScript error: resource://gre/modules/XULStore.jsm, line 66: Error: Canât find profile directory.
1589978469736 Marionette INFO Listening on port 56800
1589978470263 Marionette WARN TLS certificate errors will be ignored for this session
1589978470265 Marionette INFO Proxy settings initialised: {âproxyTypeâ:âdirectâ}
mei 20, 2020 2:41:10 PM org.openqa.selenium.remote.ProtocolHandshake createSession
INFO: Detected dialect: W3C
2020-05-20 14:41:10.658 INFO c.k.k.core.webui.driver.DriverFactory - sessionId = e8a00148-0a6c-4e7a-b22e-f327c6710143
2020-05-20 14:41:10.695 INFO c.k.k.core.webui.driver.DriverFactory - browser = Firefox 76.0
2020-05-20 14:41:10.696 INFO c.k.k.core.webui.driver.DriverFactory - platform = Windows 10
2020-05-20 14:41:10.697 INFO c.k.k.core.webui.driver.DriverFactory - seleniumVersion = 3.141.59
2020-05-20 14:41:10.698 INFO c.k.k.core.webui.driver.DriverFactory - proxyInformation = ProxyInformation{proxyOption=NO_PROXY, proxyServerType=HTTP, username=, password=*****, proxyServerAddress=, proxyServerPort=0, exceptionList=}
JavaScript error: resource://gre/modules/NetworkGeolocationProvider.jsm, line 555: TypeError: xhr.response.location is undefined
It does show a bunch of warnings and at the end a JavaScript error, but donât think those are related? After that my tests loads fine up until where I expect my download to be automatically downloaded. Yet Iâm blocked by that firefox popup

I tried to go and check that temp profile mentioned in the console log
C:\Users\ABC~1\AppData\Local\Temp\rust_mozprofile.KwcXF9b8qprj and indeed, I do not retrieve the prefs settings there in the prefs.js file. Might that be a clue my custom profile is not applied?
Sorry for the long post, but I tried to be as detailed as possible. Any help here would be greatly appreciated!!!