webpage not opening in firefox 3.0.8 in ubuntu 9.04

I've got a weird problem. My network is windows PPPoE. I can access it by giving username & password in my ubuntu Jaunty. The network status shows connected.

But whenever I try to update a program ubuntu shows failed. Also when using firefox 3.0.8 I can go to yahoo, msn, mozilla.com. But no page loading for all other websites: aol, somewhereinblog.net, amarblog.com, msnbc, cnn, ..............nothing seems to be loaded. If I ping those sites, response is normal. (There's no proxy in my network that I've to configure in firefox).

I can easily access all the mentioned websites using firefox 3.0.10 in win XP.

However my ISP told me there is no problem with the connection.