Benchmarks, in my opinion, do a lousy job of conveying real-world experience, but a test on memory use from PC Pro seems worth repeating:
"Firefox is still disappointingly inefficient at handing back memory. With five tabs open the browser chomped through 104MB of RAM in our tests - significantly less than either IE9 or Chrome - yet yielded only about 35MB of that when we closed all but the Google homepage. Both Chrome and IE9 are much more effective at releasing RAM when tabs are closed, so if you use a lot of tabs you'll still find the need to close Firefox down every so often to start from scratch."
Still, most critics didn't have any glaring complaints with Firefox 4's performance, and said the browser easily beat its predecessor. "In hands-on experiences, one of the best performance differences between Firefox 3.6 and the current version is that Firefox 4 crashes far, far less," CNet's Seth Rosenblatt wrote.
As Rosenblatt notes for CNet, Firefox 4 adds HTTP Strict Transport Security, which tells the browser to automatically create a secure connection when logging into a website. The new Content Security Policy is designed to block cross-site scripting attacks.
But the most publicized security feature is Do Not Track, which attempts to tell websites not to follow you around the Internet, thus preventing them from sharing your browsing habits with advertisers. "The problem is that websites don't have to honor this request, thus rendering the tracking protection feature useless," Nick Mediati writes for PCWorld. "Mozilla is working to make this feature an industry standard, so hopefully things will improve in time."
The browser you choose is, of course, a matter of personal preference, but here's what the critics have concluded about Firefox 4:
"The new versions are done and dusted, and the bad news for Microsoft is that you still have to look beyond Internet Explorer if you want the best browsing experience. That leaves you with an enviable choice: Google Chrome if outright speed and performance are a priority, or Firefox 4 if a more fully featured browser is what's called for." - Barry Collins, PC Pro
"For those who frequently keep many tabs open and want a way to tame them, it's clearly the best browser out there. But even aside from Panorama's capabilities, Firefox users and those who may have stayed away because of Firefox's cluttered interface will want to give it a try, because of its increased speed and clean interface -- and because Firefox still has the largest collection of add-ons of any browser." - Preston Gralla, Computerworld
"Some people have probably abandoned the browser for the significant speed differences between version 3.6 and Google Chrome. However, the competition has forced Mozilla and others to put out better browsers in order to thrive. Firefox 4 is arguably the best browser on the market today." - Seth Rosenblatt, CNet
"So is Firefox 4 good enough to lure back old users and to keep existing users satisfied? For me, the answer is yes. While I don't anticipate using Firefox as my primary browser (I tend to use Safari), keeping Firefox running on my computer is no longer something I fear." - Christina Warren, Mashable.