David, thank you. I think with your help, I have it fixed. I only had one prefs.js. (I had to go look for the file.) Anyway, just the one file. It was dated in 7/x/2023. No files named prefs-1.js or ........ prefs-46.js. So I checked the properties of the prefs-js file. It was marked read only. I unmarked read only and closed Thunderbird and reopened it. Twice now. The account I wanted to delete did not reappear and the account I wanted to add came back!! I am very pleased the solution was so simple but I would never have found it without your help and clues.