Linuxmint 21.2 Cinnamon
Where is the Thunderbird address book located in the file system?

It should be in a file named abook.sqlite
It's located at ~/.thunderbird/<some random string>/abook.sqlite

Regards

What do I read t with?

Based on the file name, some sort of SQL engine, that is, a program to parse SQL databases.

If you want to open it, it might be a good idea to make a copy, then work with the copy.

Use sqlite3 to export it in sql:

sqlite3 ~/.thunderbird/svyc3qyi.default-release /abook.sqlite .dump > address.sql
Error: unable to open database "/home/tom/.thunderbird/svyc3qyi.default-release": unable to open database file

also tried:
~$ sqlite3 ~/.thunderbird/svyc3qyi.default-release /abook.sqlite-wal .dump > address.sql
Error: unable to open database "/home/tom/.thunderbird/svyc3qyi.default-release": unable to open database file

I had to install sqlite3. After the above attempts I restarted the computer but still got the same results.

There is a blank space after svyc3qyi.default-release in the path to abook.sqlite above (i.e. /home/tom/.thunderbird/svyc3qyi.default-release /abook.sqlite).
I don't know if it should exist there, so check carefully if you use the correct path to abook.sqlite.

Also it could be that you don't have permissions to read that file.
What gives:

tom@tom-iMac:~$ ls -l ~/.thunderbird/svyc3qyi.default-release/abook.sqlite
-rw-r--r-- 1 tom tom 262144 Dec 23 12:20 /home/tom/.thunderbird/svyc3qyi.default-release/abook.sqlite

tom@tom-iMac:~$ sudo sqlite3 /home/tom/.thunderbird/svyc3qyi.default-release/abook.sqlite .dump > address.sql
tom@tom-iMac:~$
Nada!

tom@tom-iMac:~$ sudo sqlite3 /home/tom/.thunderbird/svyc3qyi.default-release /abook.sqlite .dump > address.sql
Error: unable to open database "/home/tom/.thunderbird/svyc3qyi.default-release": unable to open database file

No nada! That worked!

Check the file address.sql created in the directory from where you ran the sqlite3 command.
Apparently the blank space in the abook.sqlite path is the culprit.

without space:
tom@tom-iMac:~$ sqlite3 /home/tom/.thunderbird/svyc3qyi.default-release/abook.sqlite .dump > address.sql
Nothing returns

With space:
tom@tom-iMac:~$ sqlite3 /home/tom/.thunderbird/svyc3qyi.default-release/ abook.sqlite .dump > address.sql
Error: unable to open database "/home/tom/.thunderbird/svyc3qyi.default-release/": unable to open database file

Which space? / abook.sqlite or abook.sqlite .dump ?

Nothing returns, means that the command output is written in the address.sql file!!!

If you want to see the output just omit the redirection:

That worked.

Of course it did!
I thought that you knew the meaning of ">", i.e. redirecting a command output to a file

Anyway, you can mark the thread "Solved" from the "Thread Tools" on top of the page.

Cheers

No, I did not know the meaning of > . was going to ask. It directs command output to a file. A | directs output to another command.