I was unaware of that autocomplete attribute and you are correct, it should probably be 'off' be default on the wwAutocomplete control. However, I just implemented it and it does not work in FF 11 (but does in IE and Chrome). I tried it in the input control as well as at the form level.
Firebug confirms autocomplete is "off" at the form level as well as the specific input control. However, the history of that textbox continues to display. Only when I set Tools - Options - Privacy - Custom Settings - Remember Search and Form History to "off" will the history truly be "off" such that the drop down list of prior entries does not display.
I didn't see anything on the net about this being a FF problem, but will continue to look.