It works fine for me(in fact I am using firefox right now), maybe a little reset to default settings will do the trick or try clearing browser history and cache. If it is still not showing the captcha with the same error message, maybe the problem occurs because of ISP or VPN to and the only way to solve it is to change your ISP or by disconnecting your VPN, do not use it to connect in bitcointalk (if you are using one).