Thunderbird performs a compaction. When it is finished the blue progress line is still showing on the bottom right hand side of the window. You can select any folder and any view condition (read, all, tags etc) and no emails are shown anywhere. * Restarting Thunderbird fixes the problem with no errors. * This has happened a lot of times now and is consistent. * If you force a compaction the problem does not occur. Update: Usually, in frustration, I restart Thunderbird to fix the problem. This time, whilst typing this forum question, the problem resolved itself. I guess about ten minutes elapsed before the application righted itself. I will watch for the next incident and see what other information can be observed.