See this topic about repairing a corrupted address book:

https://support.mozilla.org/en-US/questions/1283037

Thanks for help. The "Tools/Export" function does not work. In general, the reply to solve a corrupted Address Book is beyond my technical capability. Any other ideas? Thanks all.

Export doesn't work? Are you using TB 75beta, where export is broken? Make sure to not select All Address Books when exporting.

Try this: create a new address book from File/New..., name it and close it, then drag a contact from Personal Address Book, while holding the Ctrl key, and drop it onto the new address book icon. Are changes to the contact in the new book retained between TB restarts?

I found the following suggestion on the Internet and it worked!

Method 2: Repair MAB File by Renaming abook.mab.bak file

To begin the process to repair MAB Files you need to follow some step-by-instruction that will help you to rebuild MAB File contacts: Go the folder where mab file are saved: Press (Windows+ R) command. Enter the command: %APPDATA%

Go to Thunderbird\Profile\Profile_name Find the File namely abook.mab.bak. If you are unable to find the file, you will need to make the files visible from the hidden mode. To make hidden files to Visible mode:Go to View Tab on the Menu bar Click on the " Apply " button. Now Rename the abook.mab file to abook.old.mab. If you are unable to rename the abook.mab file then create a new Folder & place the abook.mab file into the folder. (Note: Make sure to make a copy of abook.mab.bak file and then rename the copy of the bak file to abook.mab)

That is the same as I what I pointed to in my first reply, without the benefit of recovering the existing contacts in Personal Address Book.

Thank you very much. My computer skills are limited and am grateful for a solution.