Mozilla has released Firefox 6 as last stable Firefox version. There is not much major different on interface and feature between Firefox 6 and Firefox 5. But you may surprised if you look at the change log that shows 1,300 bug fixes (which includes feature and developer tooling adds). What a big change Web addresses now have the site domain name in a darker font than the rest of the extraneous parts of the address. This version has faster startup time if they have a bunch of Panorama tabs loaded. Panorama allows users to organize tabs into groups. Having Firefox launch several groups on startup has been a sure-fire way to bog down that process in the past.
Firefox 6 also includes a new site-specific privacy settings page, which you can access by typing about:permissions. The site identity area on the left side of a Web page’s address has also been slightly redesigned. With this feature, a user can configure such things as acceptance of cookies, location sharing, pop-ups, passwords and data storage.
How to Upgrade Firefox
You can upgrade your firefox by download, install and it will replace your old firefox version with the new one. Another way is by update from Firefox. Click Help -> About Firefox -> Check for Updates. It will also update automatically if you set automatic updates on Option > Advanced > Update
Please note you will notice that some add-ons will not work when you upgrade your Firefox. What worked just fine in Firefox 4 may well not work at all in Firefox 5 or 6.