PCWorld posted 4 reasons why you should upgrade to Firefox 6

The first reason is speed, although that point is debatable. According to Mozilla, startup times with Firefox 6 are better than its predecessor, especially if a user is using Firefox's Panorama feature. 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.

Any speed improvements over the sluggish performance of Firefox 5 is welcome news for users, but overall performance of Firefox's latest incarnation appears to be more problematic. For example, Dan Howley pointed out in Laptop magazine that Firefox 6 is actually slower than its progenitor when evaluated with a benchmarking program called Peacemaker.

  Firefox 6: 4 Reasons to Upgrade