“The default button to move emails into the archive is [a]. The
archive will be divided by years only by default which could be
problematic for accounts that receive many emails on a regular
basis.“The only available option to customize the archive is the
advanced config editor which makes it possible to change the
archiving granularity, names and other settings. The Thunderbird 3
add-on Message Archive Options is basically a frontend for the
advanced config options.”