In Firefox 1.5, choosing a different value for this select causes a vertical scrollbar to appear for the application window.
The problem also occurs if the user has only one tab open to this page, has the preference "Hide the tab bar when only one web site is open" checked, and opens a page in a new tab (or even just creates a new tab and returns to this window.
Some kind of reflow is triggering here.
The problem is in the style code:
<table style='margin-left: 10em; margin-right: auto;'>
If we remove the "margin-right: auto;", the problem goes away.