Rated 4 out of 5 stars

Very nice tool - many thanks! Export to vcard should be a default feature of Thunderbird. This add-on finally adds that capability.

Some feedback:
- Although the sync succeeds I get this error when exporting/importing to/from a single file: A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.
Script: chrome://thundersync/content/ThunderSyncVCardLib.js:402
(where the line number is different every time)
I just stop the script to continue.
- Would be nice if one could only sync e.g. accounts which have phone numbers. No need for all my contacts to be in my phone.
- It's strange you have to select an *existing* file when exporting to a single file: it might be the first export, so a file would not exist yet.
- Sync of contact goes wrong when displayname contains = (equal sign).
- Sync of contact goes wrong when email contains " (quote).

This review is for a previous version of the add-on (2.1).