It seems to have been around forever, but Firefox 3 is finally on the way out - and this is the final official release of Firefox 4.
The most immediately obvious changes come in the interface: the tabs now appear at the top of the window, very like Chrome, and the menu bar is replaced by a single Firefox button. It's all very clean and clear, with the maximum possible amount of space being given to whatever web page you're viewing.
New HTML5 and CSS3 support should mean those pages will look better than ever, and there are a number of other features that will enhance the browser in the future. WebM support, for instance, allows you to watch HD quality video online. WebSockets will help developers build gaming, chat and other real-time applications. And Indexed DM allows developers to store data locally, so apps like Gmail can work even when you're not online.
Then there's the new Add-Ons Manager, which takes up a full tab on its own and makes it easier to manage your extensions.
A range of privacy fixes help to prevent sites from uncovering your browsing history.
Note that this is the final official release of Firefox Portable 4.