Originally Posted by StuartD

There can be quite a few reasons for that to happen, from faulty cache to dns issues.

However, I think one main reason may be ipv6. So try this and see if it helps:

In the address bar, type in "about:config" and press enter and confirm it.

Now search for "network.dns.disableIPv6" and change it from false to true by double clicking on it.

Now restart FF and see if that makes any difference for you.