I found the trouble. Firefox 4 somehow dug up a week-old version of the menu page from the cache. The urls of those pages were different in the old version. Yet, FF 3.6 had displayed the new version correctly for a week.
I found the trouble by viewing the source. It didn't match my new code.
Refreshing the page fixed it. I didn't think of it, because the FF 3.6 had been displaying the new page for a week.
Now, how do we keep software companies from changing your settings when they upgrade your software? I found automatic checking for new versions of web pages disabled.