Disable firefox DNS prefecthing

1. Type about:config in the address bar, press Enter.
2. Context click (right-click) in the preferences list and select "New -> Boolean". [right click on the bar which says preferences list]
3. Type or paste in the preference name network.dns.disablePrefetch and click OK.
4. Select true as the value and click OK.

If this doesn't help you can re-enable DNS prefetching by resetting the preference to "false" (double-click to toggle).