Unified complete feature, which was released with Firefox 43 and currently exists in version 46, can be disabled using the hidden preference — browser.urlbar.unifiedcomplete — in about:config. FYI, when you start to type in awesome bar, first suggestion appears with search icon, that popup was because of that feature.
Since the feature was already landed and being active for four releases, Mozilla has decided to remove the unifiedcomplete preference and the underlying code for that from the Firefox browser, target milestone for this was set for Firefox 49.
Disable Firefox’s Unified autocomplete feature with Classic Theme Restorer add-on
Don’t worry! Classic Theme Restorer add-on developer version has a fix in place for this, and lets you disable unified complete preference.
In dev. version of the add-on, visit Location bar (2) settings of add-on options and under ‘autocomplete’, you’ll find Disable unified autocomplete feature (about:config preference) setting with a checkbox, check it and click ‘OK’ to turn off the feature.