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:
- Type about:config in the Location Bar
- Right-click on the page and create a new boolean value
- Type browser.urlbar.autoFill
- Set the value to true
Note: Inline autocomplete is enabled by default in Firefox 14 and above.  To disable it, set browser.urlbar.autoFill to false.
Related bug reports
- Bug 566489 - Enable inline autocomplete again, but make it smarter (perceived performance)
- Bug 735187 - Re-enable inline autocomplete once its blockers are fixed
- Bug 740076 - autoFill should be disabled by the autocomplete preference