Created attachment 619248 [details] error console log Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:13.0) Gecko/20120425 Thunderbird/13.0 BuildID: 20120425154248 (I.E. first TB 13 Beta1 on Snow Leopard) After updating on the beta channel, I have large black borders around tabs. Here's what it looks like in safe mode in a new profile: <http://ilias.ca/screenshots/tb-blackborder.png>. I tried to find a regression range on comm-aurora, but all comm-aurora builds I tried did not have the issue. I tried renaming the Thunderbird app to Thunderbird-old, and installing from disk image offered at <http://www.mozilla.org/en-US/thunderbird/channel/>, but it still displayed the border. There were some CSS warnings in the error console, which I'll attach to this bug. I see something similar on Windows XP, but it's more of a shadow, so it's hard to tell if it's intentional or not on Windows XP.
This was caused by a backout in mozilla-aurora just before uplift.