In Firefox under options check the advanced tab. In the general tab there’s a box to uncheck “Search for text when I start typing”
Under Privacy there are boxes to uncheck.
I don’t know if this will help you, or if you have done this already.
I think those items that appear in the Google search box have nothing to do with any searches you have done. When you start to type in a search, Google uses the data from all of the searches people make to predict what you are likely to be searching for.
Just now, out of curiosity, I started typing "what is the" and the top query that Google spelled out was "what is the cinnamon challenge?" That most definitely is nothing I've ever searched for before! After making that search, Google then assumed that I wanted more cinnamon challenge searches, and populated the box with those after I had typed a couple of letters. I think, in that case, it was looking at cookies (which you can delete along with the history).
I've never found a way to completely clear a single box in a single operation, but if you highlight the entry you want to remove, you hit the "Del" key, and it goes away. This works in Firefox, not sure about other browsers.
I use this to delete, histories,cookies, almost anything that is tracked.
Index.dat Analyzer
Index.dat files are hidden files on your computer that contain all tracks of your online activity, where have you been on internet, what sites you visited, list of URL-s, files and documents you recently accessed. Index.dat files stored on your computer are obviously a potential privacy threat as they can be found and viewed without your knowledge. Index.dat Analyzer is free tool to view and delete contents of hidden index.dat files.
http://www.windows7download.com/win7-index-dat-analyzer/download-syzxcdms.html