With the release of Ubuntu 11.04 Natty Narwhal on April 28th, it is now possible to use the Revenenue Online Service, hitherto referred to as ROS, on a default install of Ubuntu desktop in Firefox (still doesn’t work in Chromium).
Copy the certificate files into “ROS/RosCerts” (case sensitive) in your home directory. When you go to the ROS website for the first time you will get a security warning, say “yes” or “accept” and enjoy!