- define your favouring filters and mark the mails with one of the Labels available in Mozilla or Thunderbird (Important, Work, Personal, TODO or Later)
- create a search on your account using as a rule Label is <what_you_want>
- save the search result as directory
One of the nice features of Outlook is the Follow up directory. As I am using for a long time and I don't want to switch from Mozilla or Thunderbird, I was looking to see this feature in it. The following trick will give you something very near the Outlook feature, but unfortunately not as nice as that one.