I know what you're talking about with respect to the search entry in the URL bar list, but I don't know any way to replicate it with Firefox. However, I can suggest this:

Go to Google (or whatever search engine you like) and right-click in the text entry field. Select "Add a Keyword for this search". It'll pop up a dialog that asks you to enter a keyword and a name. Enter an appropriate name (it's just a descriptor) and then enter a short keyword. (I use "g".) Then you can type into the URL bar "g search items" and hit return and it'll perform that search for you. I find that even easier than the thing you're describing.