Recently I started having a problem where my Firefox would take 10-20 seconds to resolve each domain, this was extremely frustrating! After digging through my linux install I was unable to recreate the issue in any other program and finally blamed it on firefox.
Well after lots of searching I found this.
Firefox Slow DNS issues
The Fix:
Type this in your firefox URL bar.
about:config
Type this in under the filter:
network.dns.disableIPv6
Then double click the option and it should turn to “true”.
Magically firefox is no longer slow!