Application data in Thunderbird is used to store your e-mail address, your settings, any extensions and other data the program uses.  For Thunderbird, it reads it as your User Profile data.  When you have multiple Profiles, you can load each one.  These are saved in separate folders in a hidden area of your computer.  While most computer users would never even know how to find this information, it is accessible with a little bit of work.

First, you have to make hidden files and folders accessible.  To do this, you want to open up any folder on your system.  Go to Tools, then Folder Options and click on the View tab.  Once in the View tab, scroll down and check Show hidden folders and files.  Once that is clicked, you will be able to see hidden data.  Next, you want to go to My Computer and open it.  Click on C:\ and navigate to Documents and Settings.  Once there, go to your primary account login.  In most cases, this will be the username in your Start Menu.

When you are in that folder, you will see a folder called Application Data.  This is normally a hidden folder, once inside you will have full access to its contents.  Find the Thunderbird and copy the Profiles folder to your desktop.  Finally, once you have reinstalled Thunderbird, you simply head back into that Thunderbird Application Data and copy and paste the Profiles folder over the old one.  When you open Thunderbird, the Profiles you have will be opened instead of the default installation ones and you will be good to go.

Importing data in Thunderbird can fail.  In most cases, it will not but sometimes extensions will cause the data not to import correctly.  Since extensions and themes do not normally change the source code of Thunderbird, simply copying and pasting the Profiles folder will do the trick.  In the event that Thunderbird does not work when you follow these steps, simply delete the Profiles folders and start from scratch.