What's new in Firefox 3 :
Security :
I think that the previous reported vulnerabilities in Firefox made them more determinant to improve the security of Firefox users. The most important improvements in this issues were the the protection against cross-site JSON data leaks,anti-virus integration, malware protection, add-ons version check and the one click site info.
developers added new buttons to save time when bookmarking. The ability to put some tags about the page you're bookmarking. You can also search your bookmarks through a Places Organizer.Tabs will also be easier to locate and better integration with Vista, Mac and Linux.
Performance :
Mozilla developers are dealing with Firefox's appetite for Memory. Around 30 memory improvements were included in the Beta 2 and developers are still working on optimizing the Firefox memory use, speed and reliability.
Related links :
SDZ news article by misterti (Fr)
All the features presented by Mozilla
The Beta 2 release notes
Firefox security add-ons