From MozillaZine Knowledge Base

There are three ways to easily edit your Mozilla Suite, Firefox or Thunderbird configuration:

1. Use the ChromEdit extension (version 0.1.1.1 is not compatible with Thunderbird 1.0.2).
2. Learn how to do it manually:
   • Firefox
   • Thunderbird
   • See also the user.js file article.
3. Do (much of) it through about:config

/* Remove autocomplete dropdown */

1. PopupAutoComplete > .autocomplete-tree { display: none !important; }

.autocomplete-history-dropmarker { display: none !important; }