Steps to duplicate, using the 3-pane or alt-3-pane mail views: 1. Set up mail and news accounts, at least one of each. 2. Have unread messages in each. 3. Start reading messages in the news account by clicking on the newsgroup and then select Next. 4. Continue selecting next until all messages are read. 5. Select Next again, and Moz will prompt you for reading messages in the mail account. Do it. 6. Moz navigates to the first unread message in mail, but doesn't display the mail text.

This sounds like another variation of bug 59638. When that one gets fixed we should verify cross navigation scenarios from/to various account types. Marking this a duplicate. *** This bug has been marked as a duplicate of 59638 ***

Mark it verified/dup.