New User Interface
Firefox 4 sports a completely different interface, some of which are from Opera and Google Chrome. Firefox 4 has some new buttons and also a unified menu which is similar to the one Opera 10.6 has. In addition to that the tabs also default to the top which is similar to Google Chrome.However, the menu bar is initially hidden and users can unhide it to go back to the older navigation found in Firefox 3.6 and below.
Firefox 4 also has a new bookmark button giving users easy access to their bookmarks with one click. You can find and navigate through all your bookmarks and folders using the bookmark button next to the search box.
New Add-ons Manager
Firefox 4 sports a new add-ons manager which integrates into the browser. However, the interface is still under construction and you cannot use the add-ons manager to browse and install add-ons yet. However, it is definitely much better than the earlier version and also has some new developer tools.
Mozilla also says that the add-ons manager UI will be changed before the final release, so don’t start falling in love with it yet.
Search and Switch to Open Tabs
Firefox 4 also has a hidden gem in the smart location bar, which will allow you to switch to an open URL by typing in the URL or searching for the title of the page. The feature is very handy if you have several open tabs and want to switch to it without navigating through all the tabs.
When you want to switch to an open tab, just start typing the URL or the title of the page and you will be shown an option to Switch to that open tab. Pretty neat.
Windows 7 Integration
Firefox 4 tightly integrates into Windows 7 by having tab previews and Jumplists. Using the tab previews you can easily preview all open tabs. In addition to that the jumplists allows you to quickly perform tasks and open frequently opened URLs.
Native HTML5 WebM Videos Support
Firefox 4 has native support for HTML5 WebM videos. WebM is an open source royalty free codec for HTML5 videos which is an alternative to H.264 and Theora codes . In my tests HTML5 videos works pretty good on YouTube.
Speed and Other Benchmarks
Firefox 4 is definitely much more lighter and faster than Firefox 3.6 and it uses much less memory than the earlier versions. However, Firefox 4 is not yet the fastest browser and it was beaten by both Google Chrome 6 (dev) and Opera 10.60 in the Peacekeeper benchmarks I ran.
Firefox also failed the Acid 3 test, but the results were far better than Firefox 3.6. This shows a lot of improvements being done under the hood. Firefox 4 was also slower than in the SunSpider JavaScript Benchmark and way too slow in the V8 benchmarks I ran (screenshots coming shortly).