Changing autocomplete behavior - Firefox
The Location Bar (also called the URL or address bar), the Search Bar, and some form fields on web pages have autocomplete dropdowns that appear with a list of previously-entered data. Starting in Firefox 14, as you type URLs into the Location Bar, Firefox also automatically completes the URL (called "inline autocomplete") in addition to the drop-down autocomplete list.
- To disable or enable inline autocomplete , see Inline autocomplete.
- To disable autocomplete in form fields and the Search Bar, go to "Tools -> Options -> Privacy". Under History, set "Firefox will:" to "Use custom settings for history" and remove the check mark from the box that says "Remember search and form history". For more information, see Form autocomplete at Firefox Support.
In Firefox 3 and above, when you start typing into the Location Bar, the autocomplete drop-down will show matching sites from your browsing history and bookmarks. You can modify or disable the display of autocomplete entries in the Location Bar, as explained below.
Changing Location Bar autocomplete behavior
As you type into the Location Bar, the default behavior is to display all results from both bookmarked items and your browsing history of visited pages in a drop-down list. (This feature is often called the "Awesome Bar"). You can change Location Bar autocomplete behavior in "Tools -> Options -> Privacy / Location Bar" by selecting one of the options in the "When using the location bar, suggest:" drop-down menu:
- History and Bookmarks (default) - Displays both visited pages in history and bookmarked items (no filtering)
- History - Restricts results to visited pages in history
- Bookmarks - Restricts results to bookmarked items
- Nothing - No autocomplete results are displayed
Selecting "Nothing" sets the preference browser.urlbar.autocomplete.enabled to false and completely disables Location Bar autocomplete. This also disables inline autocomplete in Firefox 14 and above. 
Selecting "History and Bookmarks", "History", or "Bookmarks" enables autocomplete (if disabled) and sets a corresponding value in the preference browser.urlbar.default.behavior.
As long as autocomplete is enabled, you can also change autocomplete Location Bar behavior using about:config, by changing the browser.urlbar.default.behavior preference value . For example, setting the value to 1 restricts results to visited pages in history, setting the value to 2 restricts results to bookmarked items, and resetting the preference restores the default value 0 (no filtering). You can also restrict autocomplete results to tagged items (value = 4) or to typed items (value = 32) or you can match only the title (value = 8) or the URL (value = 16). You can also add filters together in various combinations, such as setting the value to 3 (1+2) to display only visited bookmarks; or, setting the value to 17 (1+16) to display only visited pages with a URL match; or, setting the value to 49 (1+16+32) to display only visited pages with a URL match for typed items. See the article browser.urlbar.default.behavior for more information. 
- Deleting autocomplete entries
- Location Bar search
- Viewing the browsing history - Firefox (Deleting history items)
- Extensions that modify the Firefox 3 Location Bar behavior:
- Hide Unvisited - This shows only URLs in your History
- Old Location Bar - This returns the Location Bar to an almost Firefox 2 Behavior.
- Extensions that only modify the appearance of the Firefox 3 Location bar autocomplete
- oldbar oldbar makes the location (URL) bar look like Firefox 2 (one line per url)
- Locationbar Limit - Change number of visible results shown in locationbar dropdown when searching (with/without oldbar)
Related bug reports
- Bug 393508 - Mockup: Location Bar Autocomplete for Firefox 3
- Bug 405745 - [meta] improve url bar autocomplete results
- Bug 740076 - autoFill should be disabled by the autocomplete preference