Above is a good idea but you could try another web browser if that doesn't work, I am finding that Chrome is a little better then Firefox and I say that as a hardcore ex Firefox fan. Also if other web sites are sometimes coming up forbidden as well, turn you modem and router and anything else off and back on again. Rebooting might not be a bad idea either but I can't see that being the problem unless your entire Internet is coming up forbidden.
Basically this,
