Created attachment 8446368 [details] screenshot of issue When title bar scrolling is on (Menu -> Settings -> Display -> Scroll title bar: checked), slowly scrolling up to show the title bar will display a small white line below the right hand side dark part (where the tab count / menu overflow thing appears). Tested on data:text/html,<body bgcolor=black style="height:20000px"> Fennec 31.0 build 20140623172615 on a Nexus 5 (portrait mode).
Thanks for the report. I see this too.
It looks like the url part of the url bar is moving out of sync with the tab counter part of the url bar. (Also: un-cc'ing myself since this isn't a graphics issue)