Mozilla Firefox and XML Limitations

I wish I was informed earlier of the XML limitations in Firefox. I was racking my brain trying to figure out why my external XML entities were not working only to discover that this essential XML feature is unsupported by Firefox.

This will NOT work:

<!DOCTYPE root [
<!ENTITY sample SYSTEM "sample.xml">

How disappointing. I understand the rationale, but It’s unfortunate that this capability is completely unavailable.