This preview of the next series of Firefox builds doesn't look particularly different on the outside, but improvements in page rendering and memory usage make it much faster that the 1.5.x line. I'm also a particular fan of the built-in spell-checking for text entry boxes.
After you've installed the beta, you might notice that a number of your add-ons and extensions have been disabled as being "incompatible". You can get around this by downloading the Nightly Tester Tools add-on, installing and restarting. From there you can go to Tools -> Add-ons and hit the Make All Compatible. This overrides the version number compatibility set by the add-on's author, but will allow many plugins to work without much issue.