There is a new Thunderbird contribution on the Mozilla Developer Center that explains how to invoke address autocomplete, searching first local address books and then globally configured LDAP servers. It works with both Thunderbird 2.x (Gecko 1.8) and 3.x (Gecko 1.9). Many thanks to simon.at.orcl!
Contributing to the Mozilla Developer Center is easy. First, get an account. Next, look for questions that need answers on the how-to page. Alternatively, write a larger article on the topic of your choice (such as the new address autocomplete article) or help out with the Thunderbird 3.0 documentation effort.
And where is the link to this new contribution?
They have TOTALLY screwed up the autofill address process in TBird 3, and now I can’t undo the damage because they flagged everything somehow. I type S, and Scott is way down the list, way below people with last names that are alphabetically after Scott and way after people that I have not written to for probably a year and hardly ever wrote to in the first place. I did, at least, find a way to delete the personal address book that stores all email addresses that ever have been received or written by us. I changed its name, and the next time tbird started up it created a NEW Personal Address Book. THEN I could delete the renamed one. The priority order of addresses for the auto fill function is still all screwed up, though.
If a person ONLY has a personal address book, perhaps it is OK, but I have my categorized into lots of different address books for better management. If a person is just starting out with TBird, it may also be fine, or if one has never tried TBird 3 it may be OK, but for us people with hundreds of addresses, it is AWFUL!!!