The free Wolfram Alpha Google add-on for Firefox lets you run a side-by-side comparison of results from Google, and the new Wolfram Alpha search engine. Google matches run down the left side of your browser as always, while Wolfram Alpha results are displayed on the right, where text ads would otherwise appear.
An icon at the bottom-right of Firefox allows you to easily toggle the extension on or off, and buttons labeled "Original" and Wolfram|Alpha" located near the top-right of a search page let you switch between a Google-only or combined view. A smaller icon to the right of these also lets you open Wolfram Alpha results in a new tab. That is, if it actually finds anything.
I say that because the new 'computational knowledge engine' cum geek almanac definitely lags behind Google in raw speed, and isn't your average search engine designed to find movie times. It does, however, do a good job of finding answers to equations and statistical questions, and it can piece together responses to fact-based questions. In some ways, the Wolfram Alpha search engine competes more with Wikipedia. If you come to the same conclusion, then you may want to try the Googlepedia Firefox add-on instead; it works in much the same way.
Note: At the time of writing, Wolfram Alpha Google's relative infancy (and lack of Mozilla public review) meant that it's still considered an experimental add-on; you'll need to tick a box acknowledging this before you are able to install it. Usefully, this release does fix previous incompatibility issues with the popular AdBlock Plus Firefox extension.