Let's start with its performance numbers. The last few versions of Firefox haven't been very fast. While generally speaking Firefox still isn't as fast as the current speed-demon Web browser, Chrome 16, it's better than it has been and faster than the rest.
As usual for my browser tests, I put Firefox up against the latest releases of Chrome and Internet Explorer, 9.08, on my Gateway DX4710 Windows 7 SP1 test box. This PC is powered by a 2.5-GHz Intel Core 2 Quad processor and has 6GBs of RAM and an Intel GMA (Graphics Media Accelerator) 3100 for graphics. It's hooked to the Internet via a Netgear Gigabit Ethernet switch, which, in turn, is hooked up to a 60Mbps (Megabit per second) cable Internet connection.
In addition, I've been running Firefox 9.01 on my main Linux Mint 12 desktop and my Mac OS X Snow Leopard and Lion desktops.
On the HTML5 Test, which checks to see how compliant the Web browser is with the HTML5 Web page standard. This test was recently updated, January 1st, 2012, so I checked all the browsers again on it. Firefox came in second to Chrome 16 with 330 out of a possible 450 compared to Chrome's 373.
When it came to SunSpider 0.9.1, where lower results are better, Chrome didn't do that well. IE won one out-right with a score of 272.5 by a nose over IE's 202.6ms, Firefox came in second with 303.5ms, and Chrome came in last with 319.7.
Still, while Chrome is still the browser to beat for speed, Firefox was much more competitive than it has been in the recent past. Firefox also has some other significant security and stability improvements. All-in-all, this is the first Firefox release in a long time that I actually liked.