I cleared history, cookies and cache.
I turned off DNS prefetching.
I started Firefox in safe mode without any extensions.
I still can't access Yahoo, Google, Microsoft and Mozilla.org from Firefox.
I suspect the problem is with my ISP and how the ISP deals with Firefox version 28. I can traceroute to Yahoo, Mozilla.org and Google (not Microsoft).
Hopefully, the issue will resolve itself. If this helps, nslookup now returns IPV6 and IPV4 network addresses and Yahoo, Mozilla.org and Google use IPV6 addresses for their DNS servers.