How to tell Firefox NOT to do XSL Transformation

Hi,

I try to create an extension for Firefox that does XSL transformation
with a C++ code without using FF's internal XSLT-Processor
Transformiix.

My Extension should start when an XML document with XSLT instructions
is requested via the URL bar.

Normally when I request such an XML document FF starts the
transformation and shows me the result.

How can I tell Firefox within my extension not to do the XSL
transformation?

Since I will deal with very large XML documents it would be annoying
if FF would do the transformation, too.