“Firefox 1.5: This version of Firefox will be supported until mid-May, 2007 with security and stability updates. We strongly encourage all users to upgrade to Firefox 2.”
The “check for updates” feature of Firefox at this time does not suggest upgrading to Firefox 2. I don’t know if that is somethign that will change later or not. Currently, a my company most of the users with Firefox are running 1.5. They tend to not use it at all so they don’t upgrade until someone tells them to.