We tinkered with the OS X build of the browser on our Macbook Pro and although pages appear to render quickly, we can't quite figure out how to navigate backwards. You can pinch-to-zoom with the trackpad and scroll with the d-pad, but once you've loaded a page, it seems you need to be on a mobile device do any further navigation. Of course, as you'll see in the gallery below, the beta comes packed with the usual -- preferences, downloads, add-ons, bookmarks and Firefox Sync. If you want to give the software a whirl, hit the source link to grab the build of your choice.
Firefox 4 beta 5 for mobile hands-on