From MozillaZine Knowledge Base

This article applies to Firefox and Mozilla Suite.

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:

1. Type about:config in the Location Bar
2. Right-click on the page and create a new boolean value
3. Type browser.urlbar.autoFill
4. Set the value to true

See also

Disabling autocomplete - Firefox