Funny that the browser that has been selling so much on privacy falls victim to such a vulnerability. In any case, if a site says "this site must be viewed in Firefox" that would be a huge red flag, and all the more reason for me to leave. There aren't really any features in Firefox that other browsers don't have.
All browsers fall to such vulnerabilities -- Chrome had one in March this year. The difference is that some browsers (again, Chrome) are malicious by design instead of only by accident.