Recently I gave Firefox 4 beta 4 a try but ended up removing it. As a beta it was not bad but I had some minor problems with Adobe Flash and the benefits were not enough to get me to keep it.
Firefox 4 beta 7 is another story. On a whim I installed it on my Windows 7 PC and it’s been smooth sailing ever since. It’s fast and I don’t seem to get any web page rendering issues. I use my browser for news reading, WordPress, Facebook (okay, almost ashamed to admit that one), etc. and some of the content is mixed and dynamic. With this new beta, the only thing that is different is the menu is hidden. Other than that, the experience is very smooth.
It’s definitely not for the casual user (I am not installing it on the shared iMac) but if you want to see what’s next then give it a try.