New Visual Look and style
Firefox 4 will look differently than the current versions and it will looks little bit like Google Chrome.Application tab will also look like Google Chrome.
Better Support for HTML 5 and CSS-3
Firefox 4 will have better support for technologies built using HTML 5 and CSS 3 for advanced styling features.
Integrated Developer tool
Firefox 4 will have advanced and much powerful tools for developers to make the debugging more easier and efficient.
New Add On Manager
Management of Add ons will become more comfortable and efficient in the Firefox 4 due to the add on manager,which will come along with the Firefox 4.