Incredibly useful Rated 5 out of 5 stars
This seems like a gimmick, but I found it unexpectedly useful. You don't realize how much you miss google autocomplete until you have it in Firefox's URL bar.
As a vim aficionado, I immediately took to the one letter search engine selectors, though other people may find that feature unnatural.
Weird side effect I loved: the configuration URLs (about:addons, about:config, etc) now pop up as suggestions.