Thunderbird : Message Grouping

Thunderbird : FAQs : Message Grouping - MozillaZine Knowledge Base: "Message Grouping (Group by Sort) feature lets you organize messages in the message list into self-contained groups according to attributes such as date, sender or priority. For instance, if you set your Inbox to group messages by date, the messages will be organized into folder-like groups labelled Today, Yesterday, Last Week, and so on. You can expand or collapse each group by clicking on the small /- mark next to the group label"