Thanks to a listener’s Comment!:
All references to GCONTACTSYNC in this article are obsolete, Use CARDBOOK Plugin

This article assumes you use a something@gmail.com Email address or Google Apps or Google GSuite (can be Free for Non Profits)

That you use free THUNDERBIRD Email already set up using IMAP but realize your Contacts are not part of the clever syncing your Mail Folders use (Warning: Will NOT work with anything but Google!- No Yahoo!, Earthlink, etc!!)

So. your online http://www.gmail.com Contacts will not appear within Thunderbird’s own Address Book,
or at least not be In Sync…
You CAN see them at: http://contacts.google.com
But they are NOT the same in your Tunderbird Contacts.
Well, Thunderbird will collect the addresses of people you write or reply to, but this will NOT be the same set as the web based Contacts. SO! :

-0) If you ever Replied or Wrote to someone within Thunderbird, most likely they will end up in the address book called
“Collected Addresses”.
Start typing some part of the name on the To:, CC: or BCC: Line,
and the whole address collected should come up…
No?

0) In Thunderbird,  Search for, and reply to whoever it is you need to contact:
use: View.. Toolbars.. Quick Filter toolbar to be certain it is ON
(It’s incredibly useful anyway)
– Search for the Name of the person as Sender that you want to write to
– assuming they wrote you at least once, they will be there and you can use REPLY!

No? OK!

1) Just use gmail.com website anyway : )

2) Best of Both Worlds:
Use Thunderbird alongside: https://contacts.google.com
to get the best of BOTH worlds- Copy & Paste from that list into the TO: line, etc…

3) I use the GContactSync  CARDBOOK add on Application to pull contacts
FROM GOOGLE into THUNDERBIRD and merge them up.
You MAY and up with duplicates this way so an additional plugin is then used to remove the duplicates. A bit messy but ideal when complete. DO NOT install it from there– that’s just for Info.
In Thunderbird, click
Tools… Addons… Extensions..
And search for:

GContactSync No! since ver 78 it’s CARDBOOK

Then permit its use on your Google account
Be Vewwy Vewwy CAreful where you Go from here as you are likely to end up with Duplicates (for which there is a separate Fixer Extension).
Best is to look carefully at the GContact  CARDBOOK Sync settings on your Toolbar (after you have Re Started) and see offerings for which directions the sync is to go: BOTH ways is probably NOT a good idea at first- Set it to copy FROM Google TO Thunderbird first?

you have Backups, right?

Within Thunderbird, click Tools > Address Book.
Select the desired Address Book (s).
Note: Make sure that you are selecting a specific address book.
The selection by default is set as “All Address Books”
and exporting this way will result in a blank file.
Select Tools > Export…
Note: If you do not see the Export option, click View > Toolbars to turn this option on.

Select “LDIF” (Industry Standard that will allow you to re import)
from the Format drop-down box.
Choose where you want to save the exported file, give the file a name, and click Save.

Comments Welcomed!