Hi Josh,
first of all, thanks for the nice addon!
But it seems to be a little bug in the read-only settings:
The 'Enable fetch only mode' setting is by default unchecked, and in the extensions.gContactSync.readOnly config value is false.
But as I delete a contact in TB and sync, it still says in the log:
* The contact was deleted in Thunderbird. Ignoring since read-only mode is on.
So I checked the checkbox at fetch only mode, and then synced again. Now it did delete the contact from goggle too.
So it seems its behaving the opposite way as the text says.
This is only a minor thing, as its working now, but maybe check it if you have a little time

Thanks
Bence