An app that was working on Firefox 2 was broken in Firefox 3. An AJAX submission produced a XML parsing error when the xmlparser is set to NEKO or NONE (TIDY worked) with FF3, but FF2 works with any setting.
The problem was traced to a difference in the "Accept" header between FF2 and FF3:
Firefox 2
text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Firefox 3
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
This caused JSF-RI 1.2 to select text/html as the content type with FF3.
The fix is to add the following to web.xml:
<context-param>
<param-name>com.sun.faces.preferXHTML</param-name>
<param-value>true</param-value>
</context-param>
A bit more detail is at http://jira.jboss.com/jira/browse/RF-3738
