I'd suggest you copy or move any messages you want to save to your Local Folders, then perform a full backup of your Thunderbird profile. The following links should help understand where the data is stored and how to back it up. * https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data * https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer * http://kb.mozillazine.org/Profile_backup '''I would strongly suggest testing the recovery of your data on a separate account or system.'''