Every time I try to search via Google on Firefox, I get a Captcha message because it detects "unusual traffic."

Whenever I open Firefox on my MacBook Air and attempt to search via Google, I receive the following message from Google with a Captcha request to verify that I am not a robot. "Our systems have detected unusual traffic from your computer network. This page checks to see if it's really you sending this request." This does not happen in Safari nor when i use Google on my iPad or iPhone - all of which use the same IP address. I have refreshed Firefox and it is on the latest version. This started about two weeks ago. There is no malicious software on my computer. I have cleared all cookies and browsing history. Help!