From the readme in thunderbird:
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-- Compatibility Warning ------------------------------------------------------
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After using Firefox v4.0, Seamonkey v2.1, or Thunderbird v3.4 (or any later
version) your History and Cookies databases (places.sqlite & cookies.sqlite)
will NOT be compatible with earlier versions (i.e. FF3.6, SM2.0, or TB3.1).
A REXX script is available to restore compatibility.
* To preserve compatibility BEFORE running the new version:
- download 'dbrestore.zip' from
ftp://ftp.netlabs.org/incoming/mozilla/dbrestore.zip or
http://e-vertise.com/warpzilla/dbrestore.zip - unzip it into your profile directory
- make backups of places.sqlite and cookies.sqlite
* To restore compatibilty BEFORE running an earlier version:
- run the script from your profile directory.
* To restore compatibilty AFTER running an earlier version
(i.e. you forgot to run the script before):
- look for 'places.sqlite.corrupt' and 'cookies.sqlite.corrupt' in your
profile directory, If they appear to be the right size, remove the
'.corrupt' extension after deleting the newly-created versions.
- run the script
- if your history and cookies aren't restored, use your backup copies.
Saludos