SW_HIDE doesn't work with ShellExecute on Firefox

[sirugo]

I have an app that opens Explorer at a given URL, but in the background with no windows open:

ShellExecute (NULL, "open", "iexplore.exe", loginURL, NULL, SW_HIDE);

Works great. I now want the same app to open Firefox in the same way:

ShellExecute (NULL, "open", "firefox.exe", loginURL, NULL, SW_HIDE);

Firefox opens at the correct URL but not in the background.
Seems that SW_HIDE does not work with Firefox.

Why and is there a workaround?
Can I use CreateProcess in the same way, opening a URL?
If so - what does the complete code look like until the browser is closed or process terminated?