Microsoft 365 - Thunderbird - Backing up and restoring a profile/email
In Thunderbird, the Profile folder contains locally stored mail, preferences and settings, extensions, etc. If you have important emails saved locally, you are strongly encouraged to backup your profiles regularly to prevent loss of data.
CAUTION: You should always verify that your backed up Profile contains the desired information before doing anything that might cause your profile to be lost, such as reinstalling your OS or deleting a user account in Windows or OS X.
The DoIT Help Desk is not responsible for any data that might be lost following the procedures below.
More information
Steps are given specifically for backing up IMAP mail locally, follow MozillaZine KB's IMAP Backup instructions.
Additional information about backing up and restoring Profiles in Thunderbird can be found in http://kb.mozillazine.org/Profile_backup.