The Lite version of RadEditor for MOSS is based on RadEditor 7. I do not think that there will be any problem with it in FireFox 3.0.
If there is something that is not working, please be more specific in describing it. Please inform us exactly which version of RadEditor for MOSS you are using. In what scenario (web part or forms such as list, wiki, blog ..)?
Any additional information and a couple of screen shots will be helpful as well.
the Telerik team