Empty folder pane
From MozillaZine Knowledge Base
If you can't see the folder pane, see Folder pane not shown. Otherwise:
1. Select Help -> Troubleshooting Information. Depending upon what version you are running you will see either a "Open Containing Folder" or a "Show Folder" button. Press it. That will launch windows explorer (or whatever file manager your operating system uses). It will display the contents of your profile.
If you don't recognize what you are looking at, check that it has a abook.mab and prefs.js file and a Mail directory. If you don't see any files in that directory, see Show hidden files and folders.
2. Exit Thunderbird.
3. Delete the session.json and foldertree.json files. They will be re-created when you start Thunderbird again, using default values.
4. Start Thunderbird.
If that doesn't work: