After all, the browser is three months old. How could Mozilla be expected to support a dinosaur like that?
Mozilla has recently moved to a quicker turnaround cycle on versions of its browser and Firefox 4 has been replaced by Firefox 5. What has caught a lot of people off guard is that Mozilla will no longer be offering security updates and bug fixes of earlier versions, meaning you have to upgrade to the new (free) browser in order to have the highest security. This is because unlike big companies like Microsoft and Google, Mozilla just doesn't have the people and resources to do a whole lot of testing.
The upshot is that while rapid updates might be good for individual users, it kind of stinks for businesses who have to go in and upgrade every machine on site.