Thunderbird users may have only a handful of accounts, but they have numerous discrete serial sources of information, including mailing lists, individual newsgroups, and feeds. With filters, the global Inbox, and Forumzilla's arbitrary feed->folder mappings, users have a great deal of flexibility about where they put their messages. But the more source->folder mappings they add, the harder it is for them to remember which mail goes where, and adding mappings is challenging for ordinary users.
Thunderbird's UI for source->folder mappings is scattered across multiple Account Preferences pages and Filters dialogs. Forumzilla adds yet another dialog for managing feed subscriptions. Users need a better way to view these mappings and change them.
I suggest we add a "sources" pane to the left of the Thunderbird folder pane. The pane, when displayed, would feature a list of sources with arrows pointing from them to their folders. Users would be able to drag arrows to different folders to change where sources write their messages. And they'd be able to double-click a source to get more detailed configuration options, which could be just the existing account, filter, and feed preferences dialogs.
Changing a source->folder mapping for a feed means changing its "target folder" preference. For a mailing list it might mean creating or updating a filter. But these are backend differences; to the user, it's the same action, and it should work the same for all sources.