Firefox only: It's not exactly Google Instant for Firefox, but InstantFirefox is pretty darned close. The Firefox add-on searches your address bar typing instantly on a customised Google page. While its looks are different, the results stay (mostly) the same.
That's pretty much all there is to InstantFirefox—you type in the Firefox AwesomeBar (a.k.a. address bar), it searches, you click to jump to a result. You miss out on the keyboard shortcuts available in the web version, and you don't get autocomplete offerings while you type. But the search results, if not as styled as Google.com or Chrome results, are right there and quickly reloaded.
InstantFirefox is a free download for Firefox only. If you've got a donation or cause you want to help out, you can set up InstantFirefox to solicit donations on that cause's behalf as you spread it to your friends—a great and very kind idea from the developers. Thanks for the tip, Mick!