Beta 3 of Firefox 3 is only intended for developers and tester but it gives you a pretty good idea of the new features that will be released later this year:
- Improved security features such as: better presentation of website identity and security, malware protection, anti-virus integration in the download manager.
- Improved ease of use through: easier add-on discovery and installation, improved download manager search and progress indication in the status bar, resumable downloading, and full page zoom.
- Richer personalization through: one-click bookmarking, smart bookmark folders, and better customization of download actions for file types.
- Improved platform features such as: offline application support
- Performance improvements.
More information at The Mozilla Blog