The major changes compared to Horde version 3.3.2 are:
    * SECURITY: Fix unescaped output in the tag cloud block
    * SECURITY: Fix unvalidated Horde_Image driver name
    * Restore backwards compatibility with older Kronolith and Whups
      releases
    * Fix problems with SQL Shares and PostgreSQL
    * Support Mozilla Sunbird snooze properties

The full list of changes (from version 3.3.2) can be viewed here:

http://cvs.horde.org/diff.php/horde/docs/CHANGES?r1=1.515.2.492&r2=1.515.2.503&ty=h

(adrianp)