Exporting Thunderbird from Win7 32bit to Win7 64bit

Instead of exporting and importing, you could copy all your mail data and profile across. After copying, open %AppData%\Roaming\Thunderbird\profiles.ini in Notepad and update the Path to the directory where your profile is and ensure that Default is set to 1. If you don't have a profiles.ini file, run Thunderbird to create a new profile (you won't be using this profile later and can delete it).

Your profiles.ini should look like this:

[General]
StartWithLastProfile=1

[Profile0]
Name=default
IsRelative=0
Path=(Give The Complete Path To Your Profile Here)
Default=1