Error when using application/xhtml+xml

[XanC]

I've got a page that uses application/xhtml+xml as its MIME type, and in Firefox (at least) when I attempt to include better-dom.js, the following appears in the console:

XML Parsing Error: mismatched tag. Expected: .
Location:
Line Number 1, Column 65:

SyntaxError: An invalid or illegal string was specified better-dom.js:179