User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/534.52.7 (KHTML, like Gecko) Version/5.1.2 Safari/534.52.7 Steps to reproduce: With TB version 9.0.1, on MAC with Parallels, Win 7. Open TB and the folder pane is empty. Even "Add-ons" can't be opened. Actual results: Yesterday happened the same. After investigations I navigated to the profile folder and renamed the file folderTree.json to folderTreejson.old. Restarted Thunderbird and the folder trees were back. Today I opened, and the folder pane again is empty. Expected results: It should have kept the folder pane from my last session. And not have been empty again.
This is already known and a temporary fix is in place in bug 707329. But if you can find when the pane is cleared then please comment over there.