After Ubuntu upgrade I noticed that Firefox become too slow.
It seems that IPv6 had been enabled again.
If I type #ifconfig it shows ipv6 address on eth0 interface.
1. Disable IPv6 module from loading:
add the following line to /etc/modprobe.d/blacklist
blacklist ipv6
2. Disable IPv6 DNS lookup in Firefox:
in Firefox type about:config in the address bar;
change network.dns.disableIPv6 value to true
3. If you have a fast connection pipelining would help:
in Firefox type about:config in the address bar; filter search by pipeline
change network.http.pipelining and network.http.proxy.pipelining to true;
you can also increase network.http.pipelining.maxrequests to 20 or even more.
4. Reboot the computer