Search for (blank) with

That is the search bar offering you a choise. If you just press <enter> your default search will be used. Or, you can choose to use another search if you want.

But how do I remove it?

I am not sure.

The location bar drop-down list can show history items, bookmarked pages, pages opened in tabs, and search suggestions. You can set what suggestions show When you enter an URL in the location/address bar bar via Options/Preferences -> Privacy & Security

• Address Bar: When using the location bar, suggest:
  History
  Bookmarks
  Open tabs

For the search suggestions, look in "Options/Preferences -> Search".

• Show search suggestions in address bar results

AnimeTitties said

But how do I remove it?

Would you please try this :

Type in the address bar about:config (press Enter) (promise to be careful, if asked)

Type in the search bar browser.urlbar.

Look for the preference :

browser.urlbar.oneOffSearches

and set its value to false

Then close and restart Firefox.

'Hope that does the trick .....