In Safari the ListSearchExtender only works with ListBoxes, not DropDownLists.
In Opera the backspace key moves back a page in the browser history by default when
it is pressed on a ListBox or DropDownList. To change this behavior you can
go to Tools|Preferences|Shortcuts, edit the Keyboard setup, search for “back” and
change the mapping for “Platform Windows-Unix-MCE, Backspace” to “Backspace” instead
of “Backspace | Back”