Tools | Address Book, right-click on the address book you want to export, choose Export, in the dialog box, choose an export format that's compatible with whatever you plan to import it into. I recommend .VCF (vCard).

I am on TB 140.9.0esr (64-bit) and for me that advice only works for an individual address book. I have seven address books (long story) and would like to be able to do a backup export of all of them in one go but right clicking on "All Address Books" only offers "Default startup directory" and Tools ... Export only offers to export everything including all my messages which would be huge and as my messages are all on a well backed up IMAP server, is not what I want. Is there a way to export all my address books in one go?

I am on TB 140.9.0esr (64-bit) and for me that advice only works for an individual address book. I have seven address books (long story) and would like to be able to do a backup export of all of them in one go but right clicking on "All Address Books" only offers "Default startup directory" and Tools ... Export only offers to export everything including all my messages which would be huge and as my messages are all on a well backed up IMAP server, is not what I want. Is there a way to export all my address books in one go?

It's best form to start a new thread for your issue so previous participants aren't alerted to a matter that may have been resolved long ago. But be that as it may...

If your goal is to export multiple address books to a common format like CSV or vCard, I don't know of a built-in way to do that. I do find some commercial programs that claim to do it, but I can't personally vouch for them.

However, if your goal is to make a safely backup of them, the Address Books are stored in SQLITE database format in your Thunderbird profile. In Windows, they can be found by default at:

<C:\Users\%userprofile%\AppData\Roaming\Thunderbird\{your Thunderbird profile}> in a series of files called "abook.sqlite", "abook-1.sqlite", "abook-2.sqlite", "abook-3.sqlite", etc. You can make a backup copy of all of them while Thunderbird is closed, and they can later be imported back into Thunderbird (one at a time, unfortunately.) These files can't easily be used by other programs.