The process is a little tricky. It sometimes helps to open the csv file in a spreadsheet program and delete the columns you don't need, and to verify if the top row contains descriptions of the columns. The goal is to match Thunderbird columns and the csv columns.

Is there an example template or file of the TB expected CSV layout? I am new to TB so I'm still learning.

The only thing on found on support wasn't very helpful and it seems that TB and Outlook have different column names for the same thing.

To see the Thunderbird CSV format, I suggest having at least one entry in addressbook and highlight, right-click and export as csv. Then open in a spreadsheet program and review. If it simplifies the process for you, change any headings in the Outlook csv to use the same wording for columns.

@david - your suggestion worked great. The only thing it didn't do was import the Outlook Groups that I had created that contained 10-30 emails in one group. I will have to figure that one out.

If that is a PST file, I suggest checking with search engine for options, as I do not know Outlook. I did see this site at https://www.syscurve.com/blog/import-pst-to-thunderbird.html and this one at https://www.wikihow.com/Import-a-PST-File-to-Thunderbird but I have no idea how useful they are. However, there are some on this forum with some expertise on the topic. Good luck.