You should browse to the old disk’s profile folder for Thunderbird, which should be something like [disk]:\Users\<username>\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default\. The file is called abook.mab - copy it to the new and similar location when Thunderbird is closed.

Make sure this is actually the last profile used - the parent Profiles folder may contain more profile folders (you can check the timestamps if unsure).

See the Profiles KB article and Files and folders in the profile.

I couldn't find the required file in the old disk so I went to another computer, found the only .mab file and copied it on a thumb drive. I then copied it to the Mozilla Thunderbird folder in the program files. I restarted Thunderbird and it didn't copy all the contacts I had on the other computer. What did I do wrong? Thanks, Alex

Did you make sure the file actually contains the contacts? You can open the file with any text editor and look if it contains the missing email addresses, so that will tell you if is a useful copy. It may look like garbage, but it’s pure text.

Unless the profile folder on the old disk is damaged or lost, the last used address book file should reside somewhere in the Thunderbird profile folder. Did you make sure to search the entire disk?

I forgot to mention: in addition to the abook.mab file, which is your personal address book, there is also a history.mab file which contains the collected addresses. So in case those are the ones missing, that is the file you need to copy as well.

Also see the Moving address books between profiles article in the mozillaZine KB.

I read the file, 8KB, with notebook and I couldn't tell what I have. I copied both "abook.mab" and "history.mab" and drug and copied them into Thunderbird and still had no addresses. I have them both on a thumb drive from another computer that has the addresses I need. Still need help, thanks, Alex

Please recheck - this should just work. Be sure to copy both files to the proper profile folder in your current Thunderbird setup. Those files should absolutely contain readable email addresses in Notepad.

In Thunderbird, open Help (menu or button) > Troubleshooting Information, then click the Open Folder button. Close Thunderbird and copy the files to the folder that just opened, (overwriting the existing files or after moving them), then start Thunderbird.

The last instructions did the trick, thank you very much. Alex