can anyone recommend a good Thunderbird extension which allows for synchronizing the address book with Google mail? So far I tried Google Contacts, but something went wrong with the syncing and some contacts where deleted on both sides. To avoid this problem, one can use Google Contacts in read-only mode (it will only fetch contacts from Google, but never write to it) but then you have to import new Thunderbird contacts to Google mail manually.
Google introduced CardDav in December 2012, which allows for syncing of contacts, but since Thunderbird's development is apparently on hold this is probably not gonna be supported out-of-the-box. There are some other extensions for Thunderbird, but since synchronization is hard and a lot more complicated than: "replace newer version with older one" I'm looking for something mature and well tested.
Before someone suggests it, I know Evolution has this feature built-in. I gave it a try last week and found so many other grave bugs with the calendar and newsgroups that Evolution is simply unfit for my needs. I really like Thunderbird and want to stick with it for a few more years until I have to look for something else.