In Firefox, when using the WCompletion component, hitting up/down works on the auto suggestion, but it also moves the caret on the text input field (i.e. hitting up takes you to the first character and hitting down takes you to the last character). It makes it hard to use up/down and type more/remove characters at the same time.
Most auto completion widgets don't have this issue, e.g. http://www.pengoworks.com/workshop/jquery/autocomplete.htm