My guess is you have an antivirus software, as that is the usual reason for losing Thunderbird components. This usually happens during an update, when the antivirus intercepts the update. Something to investigate if you do use such a product, as losing files may well happen again in the future. Anyway, on this problem, the addressbook's name is abook.sqlite, and is possibly in an archived profile. You can search for that by clicking help>troubleshootinginformation, scroll down to 'profiles', click 'about:profiles, and see if there is more than one profile. If there is, you might browse it to see if your missing file is there. If found, you can copy it from wherever it is back to your profile. For now, just see if you can locate it. Then reply back.

Hi David Belated thanks for your response to my query. I did follow your advice, and found five different profiles. Very confusing… An online search threw up an app claiming to convert Thunderbird’s old “mab” address system into “abok.sqlite”. However, that didn’t work and I ended up with two identical Thunderbird apps in my sidebar, neither of which featured an address book. Another online search shows that this issue goes back many years – my trust in Thunderbird/Mozilla appears to have been rather misplaced.

By the way, did I mention that I’m using MacOS Sonoma 14.2.1?

Well, I will agree the problem has been around for years. That is, the problem of antivirus interceptions of updates. If you found several old abook.sqlite files, I suggest you look for the most recent. If you haven't upgraded in a few years, you may still be using abook.mab file. In either case,you can use tools>import>import from a file>import addressbook and import either an sqlife or a mab address book.