Firefox 4.0 has changed a lot of thing’s like:-In the Firefox 4 nightly builds, and in Firefox 4 Beta 1, they have changed the default tab position so that tabs are on top. This is a preference that users can change by right clicking on any of their toolbars. Moving the default tab position is obviously a significant and to some extent controversial change to the Firefox UI, which are really difficult to handle, for example have a look on the image below 
you can clearly see , the tabs cant be seen clearly, as per the firefox 4.0 has fixed this problem, as u can see in the image below:-
Now you can see,that the tabs are moved to the top and are easy to be used ,I have to say it looks more attractive too . Recently modern browsers have been transitioning by placing tabs on top, and that decision isn’t arbitrary, it isn’t about fashion. The change to placing tabs on top isn’t about one browser versus another browser, it’s about the evolution of the Web as a platform. Now in the image below you can see the rest changes in firefox 4.0
well it looks cool and in this post i will give u the link to download
The Mozilla Developer Preview 3.7 Alpha 5 (code name for firefox 4.0) is available for download here.
