This article works with the version, 10.9.1 (64-bit).

1. Visit Add-ons and Themes. Click +Download Now to download and save ImportExportTools NG.
   
   
2. Select Tools and click Add-ons and Themes from Thunderbird's menu bar. If your menu bar is missing:
   
   
   • Windows: Right click at the top to click Menu Bar and Mail Toolbar.
   • Mac: Move your cursor all the way to the top of the screen; Click the green circle to resize the window. 
     
     
3. Click once on the Gear to select Install Add-on from File.
   
   
4. Locate and click to install ImportExportTools NG. Click Add to add the extension.
   
   
5. Enter ImportExport in the box, Find more add-ons, press Enter (Windows) or Return (Mac) on the keyboard.
   
   
6. Restart Thunderbird once the add-on is installed.