Firefox 4 features an integrated settings and password sync utility that works across separate desktop and smartphone Firefox installs.
I used the new Firefox sync capability to carry my browsing session over to Firefox Mobile on my Android device.
The browser's Panorama tab management feature is just the ticket for reining in out-of-control tabs.
Firefox 4 sports a menu bar that's condensed to a single "Firefox" button—a significant space-saver.
The status bar in Firefox 4 is gone, but importantly, URLs still appear in the lower left corner of the interface when hovering over links.
UI Changes Optional
If Firefox interface changes, such as the switch to tabs on top, don't strike your fancy, it's easy enough to revert them.
The Add-on manager in Firefox 4 gets a makeover, sporting a more App Store-like appearance.