Threaded emails are sorted in the order received so they get appended. In other words, the first email in conversation will be the one sorted > sort ascending by date But as emails get replies etc in conversation, they get added to list and therefore will display in descending order. Hence newest will be at the bottom of that thread. Whilst this might seem incorrect to you, all conversation replies etc that you see in most forums etc are below the original or previous comment, so I do not see it as odd or wrong; just normal. I'm letting you know that this is not a bug. It's working as designed. Although there has been a bug report for this issue which is old and there's not been any real sign of changing anything because it's a lot more complicated than you may think. I would not bother trying to add any comment as developers get fed up with 'me too'. The bug forum is about passing on information about the issue and allowing developers to post info about progress etc. But for reading about the bug report etc then here is a link: * https://bugzilla.mozilla.org/show_bug.cgi?id=479969 One quick way of locating anything new is to use the Quick Filter Toolbar click on the 'show only unread messages' envelope icon and it will filter just unread emails. click on same icon again to remove that filter. Sometimes that method is a lot quicker and less frustrating than trying to work out where the new email is lurking. However, all threads containing unread mail should be underlined and if containing new unread then the top level thread should be both bold font and underlined. Sorry I'm not offering a solution which produces the desired result.