Recently when developing an addon for firefox 2/3, noticed that the preference window overlay ruins the default preferences window. It grows the height of the window and some times may cut screen elements in the bottom of the dialog.
Just aware the reason today, I missed an attribute "helpURI" for the "prefpane" element. Add this line, and the "help" button will show up in left bottom corner. Then everything is fine.
Since it looks like an irrelevant attribute to me, still think it's a bug of firefox. Filed on bugzilla.