Odd connectivity problem in Win7

This system has been stable for a long time. No errors, no unusual behavior.

This morning I discovered that my Firefox session (which was left open with multiple tabs) would not respond properly. Selecting a bookmark resulted in no activity, refresh = no activity. New tab can open, but no error message and no content.

Opened IE, same results.

IRC is communicating with no problems.

Avast (my AV) says no problems found, but not connected to the internet.

My Network and Sharing Center can map my network all the way to the internet.

So the question is: What screwed up overnight to block some but not all all internet access, and how do I fix it?

Win7 Ultimate, 64 bit.

Still no idea as to the cause of the problem, but a reboot resulted in an emergency update from Avast and the system now functions normally.

Avast's definitions are sub par and some of those anti virus programs have been known to cause problems affecting the OS. If Avast caused this then you have an additional reason to switch to better free or pay-for anti virus programs.