Disclaimer: I am already a usual Firefox user. Internet Explorer and I had a nasty divorce a few years ago, and despite people telling me how much better it has gotten, I still can't convince myself to go back. I've used Chrome, and I like it - but for some reason nothing struck me enough to warrant changing my ways. With limited interaction with Firefox 4, I think that I'll probably hang around with Mozilla.
Despite countless articles able to be written on detailed performance factors, speed tests, etc., I'm steering these comments more towards aesthetics, interface and features. Having said that, Firefox 4 is fast. The difference isn't so noticeable on my work PC but at home it is working like a flash. Even on my computer which was in the process of downloading a huge file and running an anti-virus scan, the new Firefox still seemed faster to me than the old one.
When you open the browser, the first thing you'll notice is that the top of your screen looks a great deal different. Most noticeably, your tabs are now above the address bar by default. Personally, I like the look of this. If your personal aesthetic preferences aren't pleased, you can drop the tabs back below the address bar.
Also immediately noticeable is the big orange Firefox button. This is actually a drop down menu that contains all the options that were previously spread out across the top - options, help, etc. Some users might have a miniscule problem at first locating these necessary functions, but grouping everything under one button sure looks good.
One of my favorite little features is the "pin tab" feature that allows you to shrink your open tabs into small icon tabs. You can do this to your most commonly used tabs to keep them available with out taking up tons of space. Just right click on any tab and select "Pin as App Tab."
Your address bar still quick searches your bookmarks and history but is now called your "Awesome Bar." Stupid name, good functionality. Mozilla has boasted that it "learns" about you as you use it more often, which I assume means it links more frequently visited sites at the top. Haven't been able to use it long enough to see what it is "learning" about me.
All in all, the interface is nice and compact. It all looks very clean, as bookmarks, home page, and refresh are all small icons. The add-ons browser had been updated and has a featured add-on page, mobile add-on page, and even the add-on of the month which I got as it is highly beneficial. I've never been one for excessive add-ons as I find many of them to be pointless. But if you're a person who loves to have them all, the new manager is pretty user-friendly.
A feature that doesn't appeal to me is the new tab groups. Ctrl+Shift+E pulls up a screen with all your open tabs shown as tiny previews of the pages they are currently on. You have the option to add a new tab group and lump tabs together by whatever qualifier you choose. It is a little clunky to me and I believe the "pin tab" feature discussed earlier eliminates the majority of tab confusion.
In a couple days of using Firefox 4, I haven't run into any issues that would steer me in a new direction. The new interface is smooth and appealing, and it is loading pages faster than any previous incarnation. And honestly, for the majority of internet users, those are the two most important aspects of any browser.