Try this: - click help>troubleshootinginformation - scroll down to 'profile folder' - click 'open profile. - exit TB - click the Mail folder and RENAME the smart mailboxes folder - restart TB and it will recreate the smart mailboxes and USUALLY fixes the problem. You may need to unselect and reselect unified toolbars.

Thanks for the suggestion David. Unfortunately my attempt didn't help.

• I followed the steps and scrolled down to 'profile folder' then click 'open folder' and a Mac Finder window opened with a single folder named "k7815xkh.default".
• I quit TB
• I renamed this folder changing one letter.
• I then re-opened TB and got an error message "Your Thunderbird profile cannot be loaded. It may be missing or inaccessible" (attached image) then TB quits without rebuilding/recreating the mailboxes, unfortunately.

I reverted the folder name back to the original and TB opened as normal with empty InBox and no "Choose" option in the properties. Any other suggestions, did I misunderstand the steps?

That was not my suggestion. You renamed the profile itself. Instead, there is a folder named Mail in the profile. Click that and you should see a folder called 'smart folders. Rename that one and then restart TB.

Thank you David.

You are welcome. I was glad to assist.