I had no problem with beta3. I hope that this will continue with beta4. For me, it showed lack of compatibility with all my extentions, but I had them working thanks to Nightly Tester Tools, except for Roboform, which is always a problem. It would not even uninstall!
Word of caution. This beta, again, steals default. Beta 3 did not. And, there is no question asked.
And, the default setting are in a different location: Tools|Options|Advanced|General|System Defaults.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.9b4) Gecko/2008030714 Firefox/3.0b4
"I've got a very poor sense of direction. I keep forgetting which way is forwards."
WinXP, SP3, 512 MB, 3.0.15 regular, 3.5.5 regular and portable , TB2.0.0.23, Flock2.5.2, IE8.0, SM2.0, Google Chrome2.0.x, Zone Alarm Firewall; AVG9.0, JRE1.6_17