Firefox just quit working. Why?

I have Firefox 3.5.6 running on two identical Thinkpad computers.

It has been working fine on both for several months, but today, it
just quit working on one of them with the message quoted below. Both
are wirelessly connected on the same network. No changes were made on
either one. I have rechecked that all the settings are identical on
both computers. The one on which Firefox just quit working works fine
with Konqueror. I have checked and rechecked everything I can think
to check. I tried reinstalling Firefox from Synaptic. What could
have caused Firefox to just quit working? What can I do to get it
working again?

Server not found
Firefox can't find the server at www.google.com.
* Check the address for typing errors such as
ww.example.com instead of www.example.com
* If you are unable to load any pages, check your computer's network connection.
* If your computer or network is protected by a firewall or proxy,
make sure that Firefox is permitted to access the Web.
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