8
I've seen this before when using right-aligned elements. I tried many things... like extra wrapper div tags to insulate the margins, etc., but no avail. Ulimately, I had to add more slack to the spacing between elements so that they were less sensitive to this 10 to 20 px swing of margins. (In other words, if I had 500px of space, I would load 480px of elements).
Also, this was with Firebird. I'm not sure if FireFox has the same issue. Like you, I was not having trouble with IE (which I test at IE 5.5).
What I did not try, but what might also work, is not using right-align. Also, I can't remember whether I explicitly set all of my padding and spacing to 0. That might also help. It might be alternating between default padding and inherited padding on your various refresh.
Finally, I would say avoid px font sizes if you can. Percentages are nice I think (allowing the user to control their font sizes).