In an earlier post you said:
. I have also tried using Opera, but it does not show any difference.
Do you mean that Opera is behaving the same as Firefox? If so, try loading Epiphany from mintInstall and see if it works any better.
I suspect that there may be a hardware compatibility problem. If three browsers all malfunction it is pretty well confirmed. I have seen some wireless cards or usb wireless adapters cause problems of this type in Windows when their drivers were corrupt. If your wireless card is not well supported in Mint it may be causing your problem