From MozillaZine Knowledge Base
Inline autocomplete mimics behavior in Netscape Navigator 4.x as well as Internet Explorer (when IE's inline autocomplete is activated). As you type, entries you have previously typed that closely match appear highlighted after your typed text.
To enable it, do the following:
Note: Inline autocomplete is enabled by default in Firefox 14 and above.  To disable it, set browser.urlbar.autoFill to false.
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