How to remove addresses from Thunderbird's autocomplete drop-down?

Try this:

1. Open the Address Book.
2. Select Collected Addresses.
3. Select the entries to delete.
4. Type Del to delete the entries.

Ensure no explicit entries for the obsolete email address exist in Personal Address Book.

You can find your answer here

This one is about as counter-intuitive as it gets. The trick is that you don't delete auto-complete entries from the main Thunderbird window -- you delete them from the autocomplete cache as accessible from within a new email.

• Turn auto-completion on
• Open a new email
• Within the email window, click the Contacts button (should create a pane at the left with the list of addresses)
• Delete the entry you don't want.

I've done this successfully on my machine.