I have, and found it to be an exciting change to Firefox 3, but I was forced to uninstall it earlier today because it still does not support the add-ons that I use frequently. It seems stable enough for a beta version, I wasn’t able to find any bugs during my short-term use. Of course the real test would be to find the smallest glitches as you use it for your daily surfing activities, but I have to pass this one since I cannot afford not to use the add-ons which are currently incompatible with version 4.
I only hope that the final version would be at least as fast as Chrome, as I observed Firefox to be lagging behind in terms of speed compared to Google Chrome. Certainly, Firefox 4 Beta is faster than Internet Explorer 9 Beta, but Chrome’s speed is sweet, and it is a standard that I hope Firefox can keep up with. The native sync feature is lame, I must say, compared to Xmarks (which, by the way, lives on after reports of it going bust. Woohoo!). The latter is feature-rich, and after using it for years, the same native Firefox capability is really not a good option.
Until Firefox 4 is out of beta and is already supporting the add-ons that I use will I only reinstall it again. The current stable version (3.6.11) in the meantime works great for me and still serves my needs. How about you? Have you tried the beta yet?