That's because Firefox, even though it checks for well formedness, doesn't check for validity.
The best solution is to sniff if the UA accepts application/xhtml+xml, and if so, serve XHTML, otherwise, convert it to HTML4.01 and serve it that way. One of the advantages of XHTML over HTML is that XHTML can be easily and quickly converted to HTML, while the reverse isn't true.
Yes, XML is (relatively) easy to manipulate but the point of the discussion is schema validation.
Most XML parsers are non-validating (i.e. they don't validate that the given document conforms to its schema).
To serve validating XHTML content the best solution is to use a schema validating editor (e.g. DreamweaverMX04, XMLSpy) and to ensure your server-side components are validating and dealing with errors.
The attitude at the moment is that it's not a high priority for web developers to ensure their documents are 100% valid. If you're going to do this then simply don't serve it with a mime-type of xhtml+xml.
-- tim lucas
http://www.toolmantim.com
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