Improving MathML in FireFox

I would like to see improvement for both IE and Firefox. As near as I
can make out, in FireFox the entire file needs to be valid XML. I have
a website created with asp.net 2 (with a master file) and it's going
to take a lot of extra work to get it to generate a file that will
work in Firefox. At least in IE I can use a plugin like MathPlayer; my
MathML needs to be correct, but I don't need the entire file to be
valid XML.  Why can't each browser have the built-in capability to
take MathML tags (separated from the rest of the code by the <math>
tags) and show the proper equation?