I have found many posts from users the same issue as mine, I have several users on a LAN sharing EMAIL "accounts" by pointing the Thunderbird account to a shared drive. However, They cannot share address books unless they export, manually merge, import and merge again... There is an add-on that helps, but does not provide true syncing with multiple users. Support for defining the location of address books on a shared drive was abandoned by Thunderbird's development community some time ago, but resurrecting it could support this small business community. LDAP is just too complex and expensive for us.