firefox visual bug

[Darren]

Getting an odd bleed through.  When I thought it might be my javascript I made this video.  http://www.youtube.com/watch?v=I4CFjyv39VQ
perhaps you may have some insight?

% firefox -v
Mozilla Firefox 20.0
% uname -a
FreeBSD .ed.shawcable.net 9.1-STABLE FreeBSD 9.1-STABLE #4 r249461M: Sun Apr 14 01:39:27 MDT 2013     darren at .ed.shawcable.net:/usr/obj/usr/src/sys/GENERIC  amd64
% 

Thanks =)

Darren