Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove moz-action Anything. Now do the same in the computers Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder. Linux: Check your user manual.

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Do you mean the blue highlighted bar saying that whatever you are typing will be searched using your default search engine?

Yes, Firefox 48 has a completely redesigned drop-down and the old unifiedcomplete setting no longer has any effect.

You can use an add-on or custom style rule to hide that bar. For example: https://userstyles.org/styles/122394/url-bar-tweaks-remove-visit-search-scroll-bar