Okay. After finding in several sources, I did this steps already:
First, I remove the libnss-mdns package with running this on terminal:
Then editing /etc/nsswitch.conf with gedit with running this on terminal:
sudo apt-get remove libnss-mdns
and make sure the hosts line is containing this:
sudo gedit /etc/nsswitch.conf
Then I reboot the machine and the problem is didn't solved.
Second, after rebooting the machine I opened /etc/sysctl.conf file and adding this into the end of the file:
Then I reboot the machine, and browsing with Firefox is still slow. However, I didn't check other programs.
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.default.all.disable_ipv6 = 1
net.ipv6.lo.all.disable_ipv6 = 1
Third, finally I tried to change IPv6 setting in Firefox by opening about:config and set network.dns.disableIPv6 to true. Then I tried to browse several sites and the problem is solved.
Being fun, I tried to use any network-related programs and the connection is going normal. I didn't know what step(s) did I do to solve the problem, but I say thank you for the solid Ubuntu community to help me solve my problem.
And thank you for Brandon Williams and jonobr that trying to help me solve my problem.
Sorry for my bad English.