It will be in this location: The backup may have a different profile name folder using different letters and numbers, but all should be the same on both bvackup and new computer. C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\
'profile name' folder is usually named 'xxxxxxxx.default' folder, where the x's are letters and numbers.
Image below shows location of .mab files. abook.mab is the default created 'Personal Address Book' history.mab is default created 'Collected Addresses' other *.mab files are ones you created.
You may need to make hidden files and folders visible. http://kb.mozillazine.org/Show_hidden_files_and_folders