Quickfile is an extension for Mozilla Thunderbird which initially aimed to speed up filing of messages into your mail folder heirarchy but now includes features for navigation and folder creation...
When you delete or move a message, most e-mail programs ("clients") simply hide the message and mark it as ready for physical deletion later on. The "deleted" messages are still stored in the folder - even emptying the Trash does not remove them. The hidden messages are not physically removed until the folder is compacted (not to be confused with compressing a file). If you don't compact your mail folders periodically, they can grow very large.
Adds keyboard shortcuts to change folder, move/copy messages, with folder name auto-completion (using only the keyboard). Same for tags. Also includes a system of rules to suggest a target folder for saving messages; re-creates Eudora's Alt-Click; facilitates navigation in the 3-pane window; makes it easy to change between To/Bcc/Cc fields in the composer; facilitates tag-oriented operations. All that with the keyboard only!