msgFilterRules.datをコピーしてあげるだけです。

MAC

~/Library/Thunderbird/Profiles/xxxxxxxx.default/ImapMail/mx.xxxxx.com/msgFilterRules.dat

Windows 7

C:\Users\xxxx\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default\mx.xxxxx.com\msgFilterRules.dat

%APPDATA%\Thunderbird\Profiles\

Windows XP

C:\Documents and Settings\xxxx\Application Data\Thunderbird\Profiles\xxxxxxxx.default\mx.xxxxx.com\msgFilterRules.dat

%APPDATA%\Thunderbird\Profiles\