Export to CSV
Use Thunderbird’s built-in Export
Open Thunderbird.
Go to Address Book → in the left pane, choose Collected Addresses.
Click Tools ▸ Export… (or use the gear icon → Export). or just right click and → Export.
Import CSV into Outlook
Open Outlook.
Go to File ▸ Open & Export ▸ Import/Export.
Choose Import from another program or file → Comma Separated Values (Windows).
Select your CSV file.
Choose where to put contacts (usually Contacts under your mailbox).
Map the fields if prompted (e.g.,
DisplayName→Name,PrimaryEmail→Email Address).Finish import.
Download NK2Edit
Go to the official site: https://www.nirsoft.net/utils/outlook_nk2_edit.html
Download the correct version:
NK2Edit (32-bit) for 32-bit Outlook
NK2Edit-x64 for 64-bit Outlook
Unzip the file somewhere (no installation needed).
Start NK2Edit, it will automatically load Outlook profile and autocomplete list.
Add items from address book, mark all email addresses, click on To and then on OK. After that all emails will be loaded in NK2Edit.
Save the profile, Outlook must be closed, close the NK2Edit and start Outlook and test autocomplete.