Unable to load pages in web browsers

The problem was related to IPv6.

1. In Firefox's URL bar type: about:config

2. Click the button with the text I'll be careful, I promise!

3. Enter IPv6 in the filter field. Just one entry named network.dns.disableIPv6 should remain in the list.

4. Right click on this and press Toggle. The value must be true.

After that, logout and login again, and it should work.

1. Are you sure that is a browser problem? Do you have a proxy? Check your browser Firefox -> Modify -> Precerences -> Advanced -> Connection
2. Do you have a trasparent proxy? Check with this: http://www.lagado.com/proxy-test
3. Does your ISP block http traffic? You say that ICMP works, check ftp, ssh and other protocols

Your browser is set to work offline.

For example in Firefox under File > Work offline. Untick "Work offline" to resolve this.