Have you been able to verify that the folders are still on your PC, even if not visible in Thunderbird? The article here tells you where to look: http://kb.mozillazine.org/Profile_folder_-_Thunderbird (it's the section 'files and folders in the profile' some way down the page) If you can find them I'd advise that you make a separate, safe copy of them -- in addition to backing up your whole current profile of course. There's then an add-on called ImportExportTools which says it can help you import folders to your Thunderbird account. I've never used it myself but with it you might be able to get your (saved) local folder contents back.