I'm using a website to homeschool our child that is not compatible with newer versions of firefox. Can I and install 3.6 and continue to use 6.0 independantly as well

The http://www.k12.com/ website gives me this message when I attempt to log on I get this message "Browser Requirements: K12 requires web browsers to support certain features to ensure you don't experience unexpected errors Please follow the instructions below to enable the features required by K12 before logging in. If you feel your browser meets the requirements or would like to continue to login, please click the login link at the bottom of this page. Web Browser This website currently only supports Microsoft Internet Explorer 7.0 or 8.0 (IE 9.0 is not supported) and Firefox 3.x. Your browser is currently not supported. I am currently running Firefox 6. How can I work around this issue?