problem with firefox dns look up time

Hi, I can’t explain the difference between the time it takes for firefox to lookup subdomains and the other browsers. for example, these two tests were run roughly at the same time:

chrome: [url]http://www.webpagetest.org/result/150415_EB_H58/1/details/[/url]

firefox: [url]http://www.webpagetest.org/result/150415_7B_HE9/1/details/[/url]

can anyone help me understand why this is happening?

btw, I am using dns-prefetch for both of those subdomains. thnaks