Google detected high amounts of web traffic coming from your system and assumed you might be an automated program or service. This issue isn't browser-specific but has higher tendencies to occur when: 1) You performed many Google searches within a short period of time 2) You are using a VPN 3) Your browser has privacy plugins enabled (especially on Firefox) 4) A malware has infected your system If this occurs infrequently, it shouldn't be a problem and you can simply perform the CAPTCHA to continue using Google. If you are still worried, perform a malware scan on your computer to make sure your system is safe.