Chrome, at least in the nightlies, has a feature that recognizes searchable websites such as wikipedia and Google Images when you start typing their URL, and lets you search them from the URL bar with a quick press of tab. Is there a firefox addon or feature which does about the same thing?
Simplest: right click on the search bar on the website you want to search, and select "Add a Keyword for this Search..."
More in depth: bookmark the site, go to Bookmarks -> Organize Bookmarks, select the bookmark, click the "more information" triangle at the bottom of the window. Fill in the "Keyword" field with the text you want to use to indicate a search from the URL bar.