We’re having trouble with float:left behavior with what appear to be simple <div>s on Firefox 9 (and evidently also Firefox 10). Here’s the screenshot:

The link to the currently misbehaving webpage is here. If (on the Mac) the user zooms in or out it corrects itself but on first page load it displays this weirdness, and I am stumped. Any ideas?
What’s happening is presumably that some of the floats in the line above are taller than you think. A left float will be up as high as it can and then float left as far as it can, so if that third float in the first line in your testcase is taller than the fourth and fifth one (even by fractions of a pixel!) you would get the observed behavior.