I\'ve found many tutorials for selenium in java in which you first start selenium using s.start(\"captureNetworkTraffic=True\"), but in python start()
Start the browser in "proxy-injection mode" (note *pifirefox instead of *firefox). Then you can call the captureNetworkTraffic method.
import selenium
import time
sel=selenium.selenium("localhost",4444,"*pifirefox","http://www.google.com/webhp")
sel.start()
time.sleep(1)
print(sel.captureNetworkTraffic('json'))
I learned the *pifirefox "trick" here.