Secunia reports that 37.16% of all scanned Mozilla Firefox 1.x installations are insecure
Check out the Secunia stats to left of screen in my News pane.
37.16% is an extremely high percentage of vulnerable computers. Compare that to 2.59% of all IE7 installation, 9.32% of all IE6.x installation and 4.68 of all Firefox 2.0.x installations.
The most likely cause of the extremely high FF 1.x percentage is the fact that FF did not have an auto-update mechanism until version 1.5.
It's great to see that IE7 has the lowest percentage of the lot despite being the most commonly scanned Web browser.