This extension seems to do allow exactly that:

https://addons.mozilla.org/en-US/thunderbird/addon/multi-ldap/

I wonder if it will still work with TB 60.

Only works for version 2.0-3.0 since we are on version 52 it doesn't help.

I am sorry, I hadn't noticed Last Updated: Dec. 6, 2010 !

In the comments for the extension a more recent fork is mentioned:

https://addons.mozilla.org/fr/thunderbird/addon/multildap2/

I also found the extension technical documentation:

http://adullact.net/plugins/mediawiki/wiki/milimail/index.php/Multi-LDAP_Technical_Documentation

Depending on your needs, interest and budget, you may consider either asking the original author, the fork author and/or the TB Enterprise mailing list if there is enough interest to update the extension for Thunderbird.

I've also contacted Adullact about this, will relay here any information I get from them.

I appreciate the support. We are looking for a free way or minimal monetary input. I looked at the multildap2 extension but it is also not for our version.

Thanks for clarifying. You can still contact the original authors and propose to help with testing, documentation, translation, etc. Even a small donation (if accepted/possible) can make a difference.

I havent' heard back from them yet, will come back here if/when I do.