Review for filemail by Zappie
Rated 4 out of 5 stars
Like it so far, some visual feedback would be nice
especially an column in the message list showing which (if any) folder is associated with this sender. Possibly an UI so you can inspect and maybe edit/delete associations.
The SQLite Manager (https://addons.mozilla.org/en-US/firefox/addon/5817/) does exactly what you want.
To create your own collections, you must have a Mozilla Add-ons account.