It's the perfect setup really, you can turn it off, but if you want it you can type "? <search term>" into the address bar and it'll go to default search - or use one of your engine short codes as the first word "@google hackernews"
about:config -> keyword.enabled -> false (to stop sending possible search terms automatically to your default search engine, this includes when you type a URL firefox mistakes for a search term)
about:config -> browser.fixup.alternate.enabled -> false (to stop firefox trying a DNS lookup/connection with the term you typed with www. before, .com after)