For an easy way to backup your Thunderbird, use MozBackup http://mozbackup.jasnapaka.com/ it is a backup and restore utility for Thunderbird and more.
To manually backup your Thunderbird, follow the directions below.
First find the Thunderbird data folder
- Open the Windows Start menu
- For Windows Vista & 7: In the “Start/Search” box, type in %APPDATA% , Click the “Roaming” item that appears on the menu.
- For Windows XP: Click Start/Run and type %APPDATA%, and click ok
- In the Windows Explorer window that opens, choose Thunderbird (this is your Thunderbird data folder).
Each folder in the “Profiles” folder is a profile on Thunderbird.
You can also navigate directly to your Thunderbird Data folder at the following path for Windows Vista & 7:
C:\Users\<Windows user name>\APPDATA\Roaming\Thunderbird\
Or this path for Windows XP:
C:\Documents and Settings\<Windows user name>\APPDATA\Thunderbird\
Note: The APPDATA folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose “Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders”.
Backing Up Thunderbird
- Find the Thunderbird data folder above
- Right click on the Thunderbird folder and choose Copy to copy the data folder
- Go to your backup flash drive, backup drive, CD, or DVD and Paste the copy of the data folder into it.
- Keep the Flash Drive, Backup Drive, CD, or DVD in a safe place; it is your backup copy of Thunderbird.
Restoring Thunderbird Backup
- Install Thunderbird at the new computer or fresh install
- Open Thunderbird, and then close it again.
- Navigate to the folder C:\Users\<Windows user name>\APPDATA\Roaming\ ,(Windows Vista & 7)
- Navigate to the folder C:\Documents and Settings\<Windows user name>\APPDATA\, (Windows XP)
- Copy/Paste the Thunderbird backup from your flash drive, backup drive, CD, or DVD into the folder over writing the Thunderbird folder already in the folder.
- Your restoration is complete.
For more details see: http://kb.mozillazine.org/Profile_backup

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Windows XP does not appear to have a folder “C:\Users\…” also the profiles for windows 7 appear to reference different locations than in XP
David,
Good eye, Windows XP does not have a folder “C:\Users\…”, the actual user folder for XP is in the “C:\Documents and Settings\{Windows User}\”
I have corrected it in the post above, thanks.
And Yes, Thunderbird is put in a different location on Windows Vista & 7, in the APPDATA\Roaming folder. Windows XP won’t have a Roaming folder, just the APPDATA folder.
Thanks for spotting that and your input.