Mozilla's Firefox 3.6 is the current stable release, at the time of this writing. However, we're running the Beta version of Firefox 4.
As you see in Figure 2, Firefox 4 has a slightly smaller web page display than IE 9 or Chrome 7. The toolbar menu is hidden, as with many other modern browsers, but the placement of the Firefox menu button in the upper left makes for a larger overall toolbar.
Firefox provides all the basic functionality of a modern web browser, plus some. The integrated spell checker can be invaluable. Pretty much any box or field you type in on web pages will be checked. Any misspelled words will show the red underlining. You can simply right-click the word to get spelling suggestions.
In Firefox, you can view all the passwords saved by the browser, which is great if you forget the login details to a site. You can even protect them with a master password, preventing others from seeing or accessing the list.
Firefox, like Chrome, has a synchronization feature, so your favorites and preferences will be the same across all your computers.
One unique feature I found with Firefox 4 is the ability to take screen shots of web pages from the icon in the lower-right corner of Firefox. This can come in handy for some, especially because it can capture the entire page, not just what is visible on the screen.
I also found a couple of things that could be improved in Firefox 4. When you open a new tab, for example, it would be nice if it would load your homepage or display something other than a blank page. Plus, you can't define multiple homepages to be displayed in tabs when you open the browserboth IE and Chrome offer this functionality.
Firefox lets you perform searches from the toolbar, but it would be nice if searching were integrated into the address bar rather than having to click in a separate search box. The download manager lets you pause downloads, but doesn't resume automatically after a connection interruption.
Mozilla, being an open-source-based organization, offers its browser for Mac OS X and Linux, in addition to Windows.