If you are using a Windows PC, the most noticeable new feature will be the look of the browser. Tabs have been moved to the top like in Chrome. Other changes include:
- New Add-Ons Manager: gives you more space to manage your Add-Ons, Themes and Plugins. Customizing your browser has never been easier!
- HD Video: Watch hardware-accelerated, super-smooth, HD-quality HTML5 video on YouTube using the new WebM format.
- Privacy improvements: Mozilla always puts privacy first, and this latest beta fixes flaws in some Web standards that could expose your browser history.
- Crash Protection: Experience uninterrupted browsing (now available on all platforms) – when a plugin crashes or freezes, you can resume browsing by simply refreshing the page.
- Performance: We know that performance is important. In this version, we focused on improving responsiveness at start-up and during page loads. This is just the beginning for performance improvements in Firefox 4.
Mozilla Blog