Thunderbird : Saved Search folder
Thunderbird : FAQs : Saved Search - MozillaZine Knowledge Base:
A Saved Search folder is a "virtual folder" in the sense that it merely displays a set of messages that meet the search criteria, while the actual messages remain stored elsewhere. If you select and delete a message inside a Saved Search folder it will get deleted from its actual location, but if you delete a Saved Search folder itself all of the actual messages will remain intact. Moreover, unlike a normal folder, if you modify the search criteria for a Saved Search folder its virtual "contents" will be accordingly updated. Your Saved Search folders will remain in the folders pane even after you exit and restart Thunderbird, until you delete them, thus giving you quick and convenient access to your pre-defined searches.