Symptoms there is something wrong with your ISP / router because of IPv6 can be: Firefox works badly or Firefox works OK but Opera does not (and there is no way to disable IPv6 in Opera) or if you leave Firefox open for some time and Google works but you cannot open any other pages or to make Firefox work you have to restart your PC, etc.
The easy way: if you use Firefox, you can just disable IPv6 in Firefox. Type
about:config and search for:
and toggle it to TRUE.
The ‘hard’ way: disable IPv6 system-wide: you must edit the GRUB. Be very careful, and edit it correctly or else your system might not boot!
sudo gedit /etc/default/grub
2. And search for this:
Modify it so it looks like this:
3. Now, let’s update the GRUB:
Or if you don’t use GRUB 2, do:
4. And finally, restart your system.