From MozillaZine Knowledge Base

I was going to number the items (in the list of things to try) but decided not to. That way, if someone adds a new item to the middle of the list, the whole list won't need to be renumbered. Wintogreen 07:19, 24 Mar 2004 (PST)

I agree with your reasoning. My suggestion is to emphasize that it is necessary to set the "View > Messages > All" (or recent, etc)for each folder My problem was trying to view sent messages and the setting was for unread messages so the screen came up blank. Jonathan Milne-fowler 12:40 UCT 27 June 2005.

Another possibility in disappearing mail is: Tools -> Account Settings -> Disk Space. There is a checkbox for "Always delete read messages". Make sure that isn't turned on. This should be added to the list. --Sean 01:45, 13 May 2009 (UTC)

Done. Tanstaafl 06:43, 13 May 2009 (UTC)