I don't want Firefox to display XML files internally, I want it to open Notepad++ instead.
I have had a look a the Tools\Options\Applications menu but there is no 'XML' content type.
I am using FF 3.6.2
Obviously this only applies to XML files that I click on in the browser, XML files opened from the file system do open in Notepad++.
Best Answer
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++.