Review for Folder Account by Olav Seyfarth
Rated 5 out of 5 stars
A must-have AddOn if you receive mail for multiple users or identities and you use filters to move incoming messages into different folders in one account. Newer TB versions are able to REPLY using the current folder as SENT but not to select your ID if you compose a NEW message. If you modify install.rdf (a pain text file, just set maxVersion to "3.3.*") inside the XPI file (which is just a ZIP), it also works fine with current TB nightlies (tested with 3.3a3pre).
This review is for a previous version of the add-on (3.1).To create your own collections, you must have a Mozilla Add-ons account.