Here's the code I'm using (from a Perl/Tk app); it seems to handle all of (Firefox, IE, Mozilla):

sub open_browser {
	my ($obj, $url) = @_;
#
# open the registry to find the path to the default browser
#
	my $cmdkey = $reg{ 'HKEY_CLASSES_ROOT/' . 
		$reg{'HKEY_CLASSES_ROOT/.htm//'} . 
			'/shell/open/command'};

	my $sysstr = $cmdkey->{'/'};
#
# replace the argument PH with our URL
#
	$url=~tr/\\/\//;
	$url = "file://C:$url"
		unless ($url=~/^(http|file):\/\//);

	$sysstr=~s/\-nohome//;
	
	if ($sysstr=~/%1/) {
		$sysstr =~ s!%1!$url!;
	}
	else {
		$sysstr .= " $url";
	}
	my $exe = $sysstr;
#
#	in case we get a fancy pathname, strip the
#	quotes
#
	if ($sysstr=~/^"/) {
		$exe=~s/^"(^"+)"\s+.+$/$1/;
	}
	else {
		$exe=~s/^(\S+)\s+.+$/$1/;
	}

# start the browser...
	my $browser;
	Win32::Process::Create($browser,
       $exe,
       $sysstr,
       0,
       NORMAL_PRIORITY_CLASS,
       '.'
       ) or die Win32::FormatMessage(Win32::GetLastError());

	return 1;
}