I find there are two extensions that I install for Thunderbird every time I set up a new computer. The first is Plaxo for Thunderbird, which does a pretty good job at syncing contacts. The second is Messagefaces with Gravatar support turned on, which you can find in Add-ons > Options > Online Services. (Wish it was on by default, it doesn’t really work without it.) Plaxo has an avatar feature but I turn it off because way more people have Gravatars than Plaxo avatars.