How to fix the Firefox anomaly in Karmick Koala

How to fix the Firefox anomaly:

First, in the Terminal, run:

mv ~/.mozilla ~/.mozilla_old

This removes previously installed Firefox and replace it with a pristine new one

and then :

1.Type about:config in the address bar, press Enter.
2. Find network.dns.disableIPv6 in the list.
3. Right-click -> Toggle. Set to true
4. Restart your Mozilla application and try again.

This appears to have solved the problem as Firefox is connecting just fine.

Thanks to Wojox and Actionparsnip