Because Chrome did it. And both browsers annoy me with that at work, where the status bar likes to pop up over UI elements in a few of our web based dashboards.

The Chrome way seems to be the motivation behind a lot of recent Firefox changes lately, including the amazing growing version number. Chrome is up to 11 now, with 12 expected in a few months. Firefox expects to be at version 7 by the end of the year. Would be nice to see browsers try and differentiate themselves, not all become the same thing from different groups.