ie: for text/xml, I have this...

    text/xml    Eterm -e less "%s"
[download]

and for streaming Real files i have this...

    audio/x-realaudio    realplay "%u"
[download]

When mozilla find a registered file type, it download the file into /tmp, then launch your application and pass this file as the first argument.

Here is a simple helper application that will log the filename and its content to the $HOME/helper.log file :

#!/usr/bin/perl -w

use strict;

# get the filename (/tmp/xxx)
my $file = shift;

# open a log file
open(LOG, ">>$ENV{HOME}/helper.log") or die "Oops : $!\n";

print LOG "Got file $file\n";

# open the file downloaded by mozilla
if( not open(FILE, "<$file") ) {
    print LOG "Error opening file $file\n";
    close LOG;
    exit 1;
}

# read the whole file
undef $/;
my $content = <FILE>;

print LOG "Content {\n$content\n}\n";

close FILE;
close LOG;
[download]

Another way would be to use something like Plugger, that allow you to register an application that will work like a plugin, and allow you to access the content like a stream.