How to open a Thunderbird Address Book File

What for?

Just to explore the file.....

Address books are sqlite files. So you'll need a sqlite viewer. I'm certain you'll find something when you search for it.

Something like this?

https://sqlitebrowser.org/

Exactly.

What file type am I looking for? As when I open the program and import the file it doesn't see the file?

Maybe, if you explain exactly what your objective is, we can help better.

I basically lost an email contact and its on my last back up folder in which I need to look up in order to retrieve it.

In that case, you might create a new addressbook, and use the import feature to import the backed up addressbook. Then, you can view and copy the lost email address.