You want to know what web browser's K9 Web Protection is compatible with.
K9 will filter Internet content accessed through any version of any browser, including (but not limited to) Internet Explorer, Mozilla Firefox, Safari, and Opera. When K9 is installed on a computer, it will apply the same filtering to all users on that computer, using any browser.
The K9 Administration interface is only supported for the latest versions of Internet Explorer, Mozilla Firefox, and Safari. Other browsers will not correctly load the K9 Administration site. Please check that you are using the most current version of one of those three browsers.