There are some interesting new features in the release. One of the notable changes is that the address bar has now been moved closer to the web content, just above the page.
If you have sites that you normally want open, now you can pin them on the tab bar so that they are always available. You no longer need to create a favourites group or homepage out them to get easy access to the sites.
Another feature that may be useful is that when you type the address of a site that is already open in one of the tabs, you get the chance to go that tab instead or you can open a new tab if that's the plan.
For users of Windows XP who cannot use the new released Internet Explorer 9, Firefox 4 can be installed on XP.
The ability to handle partial downloads is quite handy. One can stop a download midway and then complete at a later stage.
There is support for CCS3 and also HTML5.
A full list of the Firefox 4 features can be found here.