Does not happen on all fonts/sizes, using text zoom will show the issue on varying sizes. In certain cases the top border will disappear from the fieldset. I use Verdana 14 as my default font, which seems to be always trigger this. testcase coming up.

works in Gecko 1.7.x, broken on current trunk, bz was able to reproduce on IRC

scratch that, sorry, I can reproduce on 1.7.x now if I zoom right.

Is this at all related to bug 287624?

This might be related: Top border is missing on the second fieldset conditions: * strict-mode * fieldset border is styled * fieldset has -moz-border-radius * h2 + fieldset + fieldset * brower is using Georgia or Verdana at 16 points by default

I've hit upon this issue as well: this worked in Firefox 1.0, but is broken in 1.5beta2. This testcase is a rather minimal testcase that demonstrates the problem with just a single FIELDSET... initially I was seeing this with more complicated page only affecting the second fieldset and subsequent ones. With the attached test case, removing either of the margins or the font size causes this to render correctly. This is on Windows XP with standard DPI and font settings. Given the sensitivity to margins and sizings, the theory of a rounding error sounds plausible...

WFM :Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a1) Gecko/20060126 Firefox/1.6a1 Bug 317375, comment 132 says that that bug fixed this.