If you look on this page here in Firefox (I'm on 3.5.2 on a Mac) and scroll up and down, you should see random lines show up in the table. They are horizontal lines that almost look like underlines that go all the way across the table. Sometimes they are thin and black, sometimes they are thick and blue. If you don't see them, scroll up and down several times.
Has anyone seen this? Does anyone know of a fix?
You may also be able to see it here.
This is definitely a Mozilla Firefox bug.
Here are two bug reports that are about this error that you've found.
Some people reported that it was affected by smooth scrolling. I turned on smooth scrolling and the line stopped appearing (using Firefox 3.5.2), but smooth scrolling bugs me so I'm going to turn it off after I finish posting this.
I never noticed this error before you brought it up, but I never use the scroll bar to scroll pages. This error does not occur with the scroll wheel.
Anyway, looks like there's nothing that can be done for now. Sorry.