Can't Clear "Form and Search History" in Firefox
This happens when the item (in your are the "Form &amp; Search History") is already cleared.

Are you sure this is not your case?



You mentioned in you comment that Firefox autofills the search bar when you type a website. Do you mean the URL bar (also known as Awesome bar ) ?

If yes, then the described behavior is expected. The Awesome bar get its results from "bookmarks, history and tabs".

So if you want to remove urls that you previously typed, you have to clear your browser's history.



On the other hand, if you mean the actual search-bar:
 

Then right click on it and select "Clear Search History" or select the option you mention in the question.

On the image above the "stack" word is a search I have previously done. You probably don't have any, since they are cleared.

For more information on what each option on the "Clear Recent History..." means check this section.