(From Bug 530209 comment #0)
> Currently, the UI for changing what should be suggested in the location bar
> results is a dropdown menu with the following items: History and Bookmarks,
> History, Bookmarks, Nothing.
> 
> Unfortunately, this makes adding additional items (and therefore
> combinations of items) very awkward.
> 
> For example, bug 480350 adds tab matches to the location bar results. If
> this were to be added to the current prefs UI, the drop would have to
> include:
> 
> * History and Bookmarks and Tabs
> * History and Bookmarks
> * Bookmarks and Tabs
> * History and Tabs
> * History
> * Bookmarks
> * Tabs
> * Nothing
> 
> This is not friendly, and is likely to result in someone's head exploding.
> 
> Rather than a dropdown menu listing every possible combination, a series of
> checkboxes could be used - a checkbox for each item, enabling or disabling
> that type of result.

................
From: https://hg.mozilla.org/mozilla-central/rev/af22db534027
>  // The default behavior for the urlbar can be configured to use any combination
> -// of the restrict or match filters with each additional filter restricting
> -// more (intersection). Add the following values to set the behavior as the
> -// default: 1: history, 2: bookmark, 4: tag, 8: title, 16: url, 32: typed,
> -//          64: javascript, 128: tabs
> -// E.g., 0 = show all results (no filtering), 1 = only visited pages in history,
> -// 2 = only bookmarks, 3 = visited bookmarks, 1+16 = history matching in the url
> -pref("browser.urlbar.default.behavior", 0);
> +// of the match filters with each additional filter adding more results (union).
> +pref("browser.urlbar.suggest.history",              true);
> +pref("browser.urlbar.suggest.bookmark",             true);
> +pref("browser.urlbar.suggest.openpage",             true);
> +
> +// Restrictions to current suggestions can also be applied (intersection).
> +// Typed suggestion works only if history is set to true.
> +pref("browser.urlbar.suggest.history.onlyTyped",    false);


Note: This redesign effectively unblocks (or obsoletes) SM Bug 618619 - Add support for more browser.urlbar.default.behavior values (History and Bookmarks) to Location Bar Autocomplete.

Firefox bug references:
Bug 1041743 - Add ability to turn on/off Search Suggestions in preferences
Bug 1094580 - Sync the new browser.urlbar.suggest.* preferences (followup from bug 530209) 
[Meta] Bug 995091 - (UnifiedComplete) Enable Unified Autocomplete