I was looking at the release notes for Mozilla 1.8 alpha 4 and took notice of one of the new features in the Mail component:

Still in “alpha” state, the new Virtual Folder allows you to save a search as a folder (either a search within a single folder, or an account-wide search, or a search of a folder and its sub-folders). (bug 11051 )

So vFolders finally come to Mozilla; how long till this hits the Thunderbird builds?