I’m already impressed by the speed improvements. The browser launches a little bit faster and overall seems more responsive. Pages seem to render a lot faster, as well.

I am a bit disappointed that the Mac version, so far at least, looks much more like Firefox 3 than the wonderful mockup from before. I really like the cleaner interface, despite it’s slight deviation from the norm on OS X.

If you want to try it out, keep in mind that it is an early beta release. Beta software is generally full of bugs and glitches, as the whole point of a beta test is to find problems so they can be fixed before the final release. Do not remove your existing Firefox installation and do not use it for anything mission-critical. Also, don’t judge the application too harshly. It will likely be much different by the time of the final release.