When Firefox retrieves the script that inserts the CAPTCHA, the certificate (at least in my test) does not match so the retrieval fails silently. The certificate says it was issued for www.google.com, not for api-secure.recaptcha.net. If you want to allow the script to run anyway, go directly to the script address, which is https://api-secure.recaptcha.net/challenge?k=6Ldz0wYAAAAAAA5AEkGSWpUqI8_ir4G2YlFFTLKs then you can add an exception (I wouldn't add it permanently). Then when you reload the comment form (or registration form, which is the one I viewed), the CAPTCHA should appear. Google's pretty good at server stuff, so hopefully they will fix this soon.