Its really not acceptable to be using a web browser 1 year behind on security updates. Especially since almost everyone is using an auto updating browser now.

Once a feature has been out on all major browsers for a year than its fair game to use. Its also not wasting your time trying to develop a modern web app which has support for netscape navigator.

Going off the usage stats of the site I work on, almost everyone is on the latest version of their browser and the ones that are not always end up upgrading after a warning notice is sent out about old borwsers.