To control it you will have to go to the about:config in your address bar. If this is you first time going to the config panel then Firefox will give you a warning. Something about “voiding your warranty”, which I don’t quite get. Any ways, just click the “I will be careful button” to access the config panel.

Firefox config warranty warning.

By typing in the filter text filed you will be able to actively filter all other setting but the one you are looking for. In this case you are looking for network.dns.disablePrefetch. My installation of Firefox is an old one with upgrades added over time. For some reason I did not have the above parameter in my configuration. I’m guessing it is either because DNS prefetching comes turned on and there is no need to add a On/Off switch or it simply was not created during the upgrade to Ff 3.5. In any case, if you want to turn off the DNS prefetching feature or, like me, you want to make sure it is on, and the parameter is not there, you will have to create it. To do so right click on a white space in the config panel and hover down to New > Boolean. In the “New boolean value” text box that pops up enter network.dns.disablePrefetch then click OK. In the next box select false. The result will be a user set parameter.

Firefox user set parameter

That’s it. Enjoy faster browsing.

Source: Controlling DNS prefetching