You need to change the Awesomebar settings.

• See this link => https://support.mozilla.org/en-US/kb/awesome-bar-find-your-bookmarks-history-and-tabs#w_how-can-i-control-what-results-the-location-bar-shows-me

Or clear history

Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list
3. press the Delete key (on Mac: Shift+Delete) to remove it.

• http://kb.mozillazine.org/Deleting_autocomplete_entries

If this is about removing items that you have typed then you may have to make more effort and have to use "Forget About This Site" in the right-click context menu in the Library (if necessary then use the search bar in the Library).