Thunderbird:Collected User Requests
Some of these are moved from various discussion pages in the Calendar wiki.
LDAP write & IMAP ACL capability
A very important feature that the Mozilla family lacks is write mode LDAP access. Evolution has it and makes creating a shared address book extremely easy. Another gaping hole is lack of ACL manipulation from the GUI.
These two features alone would make workgroup mail management a reality for Mozilla. In any case they are both completions to already present features: LDAP read access is already present and so does IMAP folder ACL parsing (accessible from a tab in the properties context menu of an IMAP folder.
Before attacking calendaring I'd finish off what's half done.