Some bugs aren't very obvious. This one's mean, strange and unfixed in the FF3 beta.
It's about
this bug. A vertical pixel added every alternate line.
The testcase is as minimal as I could get it for FF 2.0.0.11, but the testcase was fixed in FF 3.0b3pre. Yes I say testcase, the bug is still there in my developmentproject, I just don't feel like skinning it down again.
So if you want to
submit a report, go ahead. I couldn't find the bug in the list, but it's a looooong list.
I reduced the bug to a combination of background font verdana, element font monospace and a combination of height specifications. As you can see in FF 2.0.0.11 (at least) with the testcase a pixel is added every other line. This effect doesn't appear in Opera or IE6. In fact, I wouldn't even know how to get that effect without specifying it on the specific lines.