Answering the prayers of potential Thunderbird switchers everywhere, a wonderful and as-yet unnamed individual has
put together a web-based vCard-to-CSV Converter for easily moving
your contacts from Address Book into either Thunderbird or Gmail. The interface and process are both conveniently
simple: export your desired group in Address Book (File > Export vCard) and feed it into this tool. You have three conversion options: LDIF (which is
Thunderbird-friendly), CSV and Gmail CSV.
After discovering the joys of Gmail (but not removing Mail.app from my Dock just yet) I'm not that interested in Thunderbird, but I fired it up just to verify that this conversion and import process is the first I've seen to truly work without a single hangup. The new LDIF file imports just fine into Thunderbird, and you won't even have to remap any fields.
[via Hawk Wings]