
While IPv4 allows 32 bits for an Internet Protocol address, and can therefore support 4,294,967,296 addresses, while IPv6 uses 128-bit addresses, so the new address space supports 2 to the power 128 address.
Here is how you can enable IPv6 in Firefox browser.
Step 1:
Open Firefox browser, and in address bar type about:config and press enter.

Step 2:
Click I’ll be careful, I promise! button on warning page and proceed.

Step 3:
In the filter search box type ipv6.

Step 4:
By default value of network.dns.disableIPv6 is false. This means IPv6 is disabled on Firefox browser. To enable, just double click and its value will become true.
Now restart Firefox browser. You have now enabled IPv6 in your Firefox browser.
This article is total BS. The setting is called DISABLE ipv6, and if this is set to false, IPv6 is enabled. To enable IPv6 in Firefox, you have to do exactly nothing.