Thunderbird:Replace Normal Folders with Virtual Folders
Basically, Thunderbird will save all messages into a single database. Each message will have a set of metadata associated with it, such as From, To, Account, Junk Status etc. The folder hierachy will be constructed from these metadata.
A new metadata field, keywords, will be provided for the user to add customized labels/tags to each message. They can add keywords by typing them in, through filters or by dragging them to a folder. Each normal folder will be associated with 1 or more keywords, but not other metadata (however, special folders, such as the junk folder, can associated with any metadata, including keywords). This basically means a particular message can appear under several folders.
I believe this system is far more flexible than the current system and it works better with database, which will be used as the storage basckend for mozilla2