User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Firefox/31.0 (Beta/Release) Build ID: 20140716183446 Steps to reproduce: Hovering the mouse over the folders in the Folder Pane, several white lines appear leaving a trail where the mouse was moved. This error happens only with version 31.0-candidate (ftp://ftp.mozilla.org/pub/thunderbird/nightly/31.0-candidates/build1/win32/en-US/Thunderbird%20Setup%2031.0.exe). Version 24.7.0-candidates works correctly (ftp://ftp.mozilla.org/pub/thunderbird/nightly/24.7.0-candidates/build2/win32/en-US/Thunderbird%20Setup%2024.7.0.exe). To test I installed thunderbird (31.0-candidate) on 4 different computers and the error always happens the same way. All computers used for testing have Ultimante Windows 7 (64-bit). One is configured with a video card from NVidia Another uses a video card from ATI And the last two use the Intel video card

All computers used for testing have Windows 7 Ultimate (64-bit).

Thank you for the bug report, this is a known issue, see bug 909820.