Recent versions of Windows come with an Easy Transfer Wizard which offers to transfer your data, documents, videos and email to your new computer. The default settings for this transfer wizard do not transfer Thunderbird data to your new computer. However, it is simple to make the changes necessary to have the software move your Thunderbird data.
Once the wizard has finished the "Checking what can be transferred" routine, a customize option will appear under the account name. Click this option to open a dialog that provides access to advanced options.
Click Advanced at the bottom of the list, which opens a file selection dialog. Expand the folder named Application data and check the box beside the folder name Thunderbird as shown below.
Click Save and continue with the wizard. Your Thunderbird data is now included in the data that will be transferred from your old computer to your new computer.
If you are moving from one operating system to another (such as changing from Windows to Mac), you must manually move your profile from one computer to the next. You'll need to first back up your profile using the instructions on the Profiles page, then follow the directions on that page for "Restoring to a different location."
In this method, you transfer messages to a Gmail account and then synchronize them to your new Thunderbird installation.