Firefox 3.0 was released with much funfare and with lots of testing. But it turns out a lot of people have problems with the latest version of Firefox, when starting and installing addons. Most perpetual problems are related to hanging up of the GUI when Firefox starts and those missing their old addons that are not working in version 3.0
Mozilla has released Firefox 3.0.1 that solves 2 bugs that are critical vulnerabilities according to the Firefox developers. But the startup problem still remains for a few guys... There will be auto-update out soon, but for all the guys who are in a bit of hurry can download the latest release in their languages from here.
The security fix is also applied on Firefox 2 and has been released as v184.108.40.206 and can be downloaded from here.