When a floating box appears directly after a header in Mozilla, the very first line of text after the float doesn't obey the float but is rendered right through the floating box.
The first floating box on a page is never affected by this bug; as you see the green intro paragraph stays well to the left of the floating box.
Bug confirmed in Mozilla 1.6 and Firefox 1.0 . In both browsers it can be solved by resizing the window. In addition, the browsers may suddenly behave correctly when there's enough text above the first header (or so it seems).
Resizing the window and then reloading it sometimes causes the bug to reappear, but sometimes it doesn't. This may have something to do with the total window width.
As soon as we add a header to the page, followed by a floating box, the first line of the paragraph after the box goes right through the floating box.
The effect does not take place when there is no header to precede the box; now this paragraph behaves as it should.
One final test with an h1 header: this paragraph misbehaves once again by going through the floating box.