0864: disable form @autocomplete

The default is false in FF54 (and also in FF55 beta), so there should be no downsides. Not sure how this will affect AutoFormFill system addon, and don't care since we'll disable that as well

user.js

@@ -483,6 +483,10 @@ user_pref("browser.urlbar.oneOffSearches", false);
  * [1] https://www.ghacks.net/2017/05/24/firefoxs-new-form-autofill-is-awesome/
  * [2] https://wiki.mozilla.org/Firefox/Features/Form_Autofill ***/
 user_pref("browser.formautofill.enabled", false);
+/* 0864: disable form @autocomplete (FF32+)
+ * [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1009935
+ * [2] https://html.spec.whatwg.org/#attr-fe-autocomplete ***/
+user_pref("dom.forms.autocomplete.experimental", false)
 /* 0870: disable Windows jumplist [WINDOWS] ***/
 user_pref("browser.taskbar.lists.enabled", false);
 user_pref("browser.taskbar.lists.frequent.enabled", false);