I now know my problem was not any fault/problem with the WA files as provided.
I changed the WA logo at the top of the 'choosegallery' and other admin pages to suit site branding (admin.css), and the replacement logo was 10px more in height than the original WA logo.
It affected the layout of the '#text-with-button' div, until I changed the overflow from hidden to visible. If anyone rebrands with a larger logo, they may have the same problem.
Firefox 3.5.5, Vista sp3.
Melissa's problem probably has another cause (although the visual results were identical).