> Is it really useful to have both of them?

No, `mode` is left over and needs to be removed.

>  I find it quite difficult to understand when we deal with only new-message-drafts and when we deal with both draft types, and I think we can do better.

I followed the approach you described here: https://bugzilla.mozilla.org/show_bug.cgi?id=919971#c16

- new message drafts are rendered in renderDrafts, which is called by renderThreads
- existing conversation drafts are just a bit of extra data added to a regular thread list item