I didn’t understand why firefox suddenly is very slow while programming and running on localhost. I’ve found this article “Fixing Firefox Slowness with localhost on Vista (or XP with IPv6)”
I’m using Windows 7 and not Vista or XP but the solution worked.
To make the change, type about:config in the address bar, locate the network.dns.disableIPv6 setting and double-click on it to set it to true.
See this URL from Dan Wahlin: