Created attachment 605214 [details]
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:12.0a2) Gecko/20120312 Thunderbird/12.0a2
After a recent update the tabs have all had a thick black border. See attached screenshot.
This was fixed by bug 713852 earlier today.
This is visible again for me with Beta 1 of Thunderbird 14 on OS X.
Separate instance as referenced in the release notes and already dealt with by bug 763394.