does you CSV file start with a heading naming the email addresses.

Please check your CSV file is formatted as a comma separated values file and not just a plain text file with data in it.

See https://thunderbirdtweaks.blogspot.com.au/2013/03/importing-csv-files.html

Where I tried to explain importing CSV files to Thunderbird.

I have it both in a csv and a txt fle. They contain nothing but the e-mail addresses in the form XXX<e-mail address> where XXX is the Display Name. I know how to delimit the addresses in any way by using find-replace. I reviewed your post but it appears that you are assuming the file to be imported contains a first and last name, which I don't have other than what's in the display name. Using Data Import I have been able to create a csv file in Excel with one row and each column containing an e-mail address in the above format. But I haven't been able to import it into Thunderbird

You need one contact per line, and the lines are split into columns at the commas (though other text delimiters are available.) That's how CSV works.

Giuseppe Verdi,[email protected]
Fred Delius,[email protected]
Maurice Ravel,[email protected]
Wolfgang Amadeus Mozart,[email protected]

You can use a copy-and-paste in Excel where you copy your row, and swap rows for columns when pasting.

If you follow the advice given by Matt, you can probably construct a display name column. And you can probably parse your display name entries into first name and last name (you don't need to do this, but may wish to do so for your own convenience when sorting and searching.) It may not be 100% accurate, but you will be able to reduce the amount of hand-editing to just the names which confound your data parsing. Here, Herr Mozart presents us the challenge of using two forenames.

You said the magic word. Excel.

Did you get the bit where I said Excel output can not be used without first passing it through notepad to correct the deficiencies of Excel?