While tweaking thunderbird, I started from the command line, and closed it after a little while. When I started it next, ALL ACCOUNTS WERE GONE!
Looking more carefully, I notice that both the address book and the list of extensions are intact, but no accounts show up; what does show up is the wizard for creating new accounts! Google discovered that these are the classic symptoms of a corrupted profile. In my case this turned out to be an easy fix: just replace the prefs.js in TB settings folder for a non-corrupted backup. It was easy because I had the backup!.
But that’s not the funny part. The funny part is the reason why I had that backup. You see, just yesterday I was working on a shell script to properly automate backup tasks, but it’s still not completed! (it just got a MAJOR boost in priority, though). No siree, regular backups were not the reason I got away with this so easily. The reason was that a couple of days before, I decided to give the TB3 beta 2 a try. But it expects its settings in a different folder (~/.thunderbird) whilst TB2 (in Ubuntu at least) expects its settings in ~/.mozilla-thunderbird). Having more free disk space that I realistically need, the simplest solution was just to make a copy of the latter named like the former. And hence the origin of the backup.
Well, and now I must excuse myself: shell scripts are waiting for me!