Rated 3 out of 5 stars

The exact same thing that happened to Ssurgul, happened to me. Uninstalling the plugin fixed the issue. I was able to sync my contacts with my gmail account.

This review is for a previous version of the add-on (0.8.13). 

kyle, this is a Lightning bug, fixed in Thunderbird 3.0.1, see:
https://bugzilla.mozilla.org/show_bug.cgi?id=398702