Slow rendering in Firefox

Problem
While surfing (visiting websites) in Mozilla Firefox, sometimes this process seems to be slower than normal (or as compared to using Firefox in another operating system).

Solution
Most of the time, this problem is caused by enabling IPv6, which is enabled by default. In most cases, you won't be needing IPv6. So, it is safe to disable it.
 * Open Firefox.
 * In address bar type: about: config.
 * Search for the key: network.dns.disableIPV6.
 * Right-click on the entry and select Modify.
 * Set it's value to true.

Optionally, try the following:
 * Search for the key: network.dns.ipv4OnlyDomains.
 * Make sure its value is empty. If it's not empty, right-click on the entry and select Reset.