Mozilla release the Firefox 5 after the successful release of Firefox 4, which had more than 200 million downloads on Windows, Mac and Linux and millions of downloads in the Android Market. Firefox 5 is the first version of Firefox released under Mozilla’s rapid-release cycle, which means that instead of seeing a new whole-number version once every year or two, you’ll now see one every three months.
According to Firefox official blog post
The latest version of Firefox includes more than 1,000 improvements and performance enhancements that make it easier to discover and use all of the innovative features in Firefox. This release adds support for more modern Web technologies that make it easier for developers to build amazing Firefox Add-ons, Web applications and websites.
Firefox for Android includes the Do Not Track privacy feature in this release, making Firefox the first browser to support Do Not Track on multiple platforms.
If you are using the extensions like FireShot, Google Toolbar and Skype extension then don’t upgrade your Firefox as some extensions are incompatible with Firefox 5.