User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:20.0) Gecko/20100101 Firefox/20.0 Build ID: 20130220104816 Steps to reproduce: 1. Close all windows on desktop. 2. Go to File > New Private Window. 3. Visit some site in a tab. 4. Make sure the Private Window is in focus. 5. Go into Mac OS X Exposé or Mission Control. Actual results: A faint horizontal line rendering above the tabs when using Exposé or Mission Control in Mac OS X. Expected results: No horizontal line rendering above the tabs when using Exposé or Mission Control in Mac OS X.
Created attachment 719097 [details] Screen Shot of Mission Control with Horizontal Line Above Tab of Private Window
I can reproduce this. F20 beta 2 and Firefox Nightly. Doesn't happen with the old Private Browsing mode from Firefox 19, so must be a regression introduced with the feature.