autocomplete_on.user.js

// ==UserScript==
// @name      Autocomplete On
// @namespace http://gist.github.com/481963
// @include   https://us.battle.net/login/*
// @include   https://wwws.mint.com/*
// @include   http://www.verizonwireless.com/b2c/*
// @include   https://login.vzw.com/cdsso/public/*
// @include   https://login.verizonwireless.com/*
// ==/UserScript==

function enableAutocomplete()
{
    var uNodeSnapshot = document.evaluate('//@autocomplete', document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);

    for (var i = uNodeSnapshot.snapshotLength - 1; i >= 0; i--) {
        uNodeSnapshot.snapshotItem(i).nodeValue = 'on';
    }
}

window.addEventListener('load', enableAutocomplete, false);