Internet Explorer is the embedded browser for the Windows platform. Firefox is the embedded browser for Linux platform (don't known about MacOS).
Firefox should also be an (the ?) embedded browser for the Windows platform.
This would be simular to Ctrl-M which opens a new mail message when you're in the Notes client and simular to IE, FF and other browsers that allow for new tab creation.
@HostName(servername) would return the HTTP hostname for the given Domino servername
e.g. @HostName("Server1/Mega") might return "megadom01.megacorp.com".