per clarification via email from bz (reviewer of previous checkin to this file, for Bug 216417), using .xht, not .xhtm in the defaultMimeEntries array (matches the XHTML entry in the extraMimeEntries array)

uriloader/exthandler/nsExternalHelperAppService.cpp

@@ -195,7 +195,7 @@ static nsDefaultMimeTypeEntry defaultMimeEntries [] =
   { TEXT_HTML, "htm" },
   { APPLICATION_XPINSTALL, "xpi" },
   { "application/xhtml+xml", "xhtml" },
-  { "application/xhtml+xml", "xhtm" }
+  { "application/xhtml+xml", "xht" }
 };
 
 // this is a small private struct used to help us initialize some