From MozillaZine Knowledge Base
If you've tried importing your Outlook contacts into Thunderbird but found that not all of the data (such as Notes) imported correctly, it might be because of the difference in the numbers of fields each e-mail program uses. You can export all of your Outlook contact data to a comma delimited text file (.csv) and then import the file into Thunderbird. You can import up to 35 fields in the Thunderbird address book for each entry. The list of fields available can be found here Import address list from text file.
- Export from Outlook to a CSV file. A CSV (comma-separated values) file contains data where each record starts on a new line (row) and for each record each field (column) is separated by a comma. Here's an example:
LastName, FirstName, Email
Doe, John, John.Doe@email.com
Doe, Jane, Jane.Doe@email.com
CSV files can be opened and saved with a text editor like NotePad as well as Excel. Opening a file in Excel will retain the rows and column structure of a CSV file but hide the commas. When saving the file as a CSV with Excel, Excel will insert a comma between each field (column)
- If possible, open the CSV file and edit it to match field locations and quantity of fields that Thunderbird will accept. If you are unable to open and edit the CSV file you can map your fields to Thunderbird's fields in the Import dialog box. However, it is a cumbersome process.
- Import into Thunderbird.
- Map Thunderbird fields to fields in CSV file.