Holy cow! This issue drove me CRAZY all evening. I finally figured it out. I realize the original question was long ago, but I want this answer on the internet, so here goes:
Instead of adding an entry for XML, I had to update the extensions list for text/plain. Here is what the text/plain entry now looks like in my mimeTypes.rdf file. The file is located in the firefox profile folder, see firefox troubleshooting page (type about:support into the navigation bar) and under "Profile Folder" click on "Show Folder" then edit the mimeTypes.rdf file with notepad++.
<RDF:Description RDF:about="urn:mimetype:text/plain"
NC:description="Text Document"
NC:value="text/plain"
NC:editable="true">
<NC:fileExtensions>xml</NC:fileExtensions>
<NC:fileExtensions>txt</NC:fileExtensions>
<NC:handlerProp RDF:resource="urn:mimetype:handler:text/plain"/>
</RDF:Description>
<RDF:Description RDF:about="urn:mimetype:handler:text/plain"
NC:alwaysAsk="false"
NC:saveToDisk="false"
NC:useSystemDefault="false"
NC:handleInternal="false">
<NC:externalApplication RDF:resource="urn:mimetype:externalApplication:text/plain"/>
</RDF:Description>
<RDF:Description RDF:about="urn:mimetype:externalApplication:text/plain"
NC:path="C:\Program Files\Notepad++\notepad++.exe"
NC:prettyName="notepad++.exe" />
Thanks for the link. I have now added text/xml and application/xml mime types to the mimeTypes.rdf file in my profile. The xml type now shows up in the Tools\Options\Applications and I've set the handler to Notepad++ but when I click on an XML link it still opens in the default xml viewer. Is the default viewer overriding this setting or am I missing something? Thanks – ChrisB – 2011-05-11T14:24:28.207