Rated 3 out of 5 stars
I have the same issue as vvv888, I also have a Intel-Mac running 10.6.6, Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b4) Gecko/20090423 Firefox/3.6 GTB5.
It gets worse the further down you scroll in a msg, and goes away once TB lost and regained focus (ie. after a redraw).
I have made a screenshot here: http://www.picdrop.net/?v=dispmua.png