Chrome has exactly the same feature. You can disable Google Instant, too, but it's actually a really useful feature.
Try to type
new com <part of title of something you read on hn today>
[0]: which might explain why not everyone is using the better browser :-P
you [tab] searches on youtube. news [tab] searches hn. I've never set it up and works with almost every site I've performed a search on.
I agree Chrome's site search thing is awesome, but how did you teach it to use the new HN one?
EDIT: Chrome settings -> manage search engines. Very impressed with the list of those it's stored automatically. Point stands that it doesn't seem to relearn though when one changes.
Safari will suggest also suggest things like Wikipedia articles and popular sites. Similarly, if I type "imdb michael mann", Safari will suggest "Search imdb.com for michael mann".
Firefox's location bar is arguably much worse than both Safari and Chrome — it won't really autocomplete anything except "search Google for this" and bookmarks/history. Safari also leaves the search standing in the location bar when you submit it, whereas Firefox replaces it with an obscure, unreadable google.com URL.
Being able to just type in the random things you remember from a url / title (I think it combines them) and have FF narrow down the alternatives, -fast, is something I haven't found in any other mainstream browser.
Another aspect is it doesn't automatically send everything you type to google (I don't worry to much but it is a nice touch.) Maybe the average user is confused by having a separate search field in addition to the address bar but I like it. (And yes, you can search from awesomebar as well, although there is no suggest feature.)