I have several email accounts and they all seem to be coming in OK to their respective Thunderbird inboxes, but since 2 days ago they don't show properly in Unified Folders. Mostly that simply looks empty, but sometimes it seems to have some content in it, but when I move the mouse over the entries they just disappear. Quitting Thunderbird and restarting it doesn't help, nor does restarting the computer (it's an iMac). What should I do now?