As noted on Mozilla's MDN (https://developer.mozilla.org/en-US/docs/HTTP_Cache) :

It is strongly encoraged to NOT USE the OLD cache API any more - nsICacheService et al.  It will soon be
completely obsoleted and removed

This affects conkeror in yhe following way : When saving documents with no mime type , it is not possible
to "View internally" or "view as Text".

I can confirm this bug. I've read the documentation for the old and new apis, but it will take deeper 
knowledge of Mozilla internals to fix this. Something that makes this more complicated, I think, is 
maintaining compatibility with palemoon.