You can download the latest nightly build for Mac OS X, Linux, or Windows, but be warned: it's alpha code. While a quick scan of the Web shows few complaints as to stability, Minefield may not be for you. It doesn't support some of my favorite Firefox extensions (like Adblock Plus), but it actually has surprisingly good support for extensions, given that it's a fast-moving project.
Feeling brave? Or simply feeling like your browser is too slow? Give Minefield a try. It's a separate install so it won't affect an existing Firefox install. You have nothing to lose but your chains.