Firefox just quit working. Why?

[Technomage]

On 2/20/10 9:00 PM, Josef Lowder wrote:
> 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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>    
Its not a firefox issue. its a dns issue. How identical are the machines 
in question?
running all the same processes? can you ssh from/to them?

I see a lot of such errors, not just with mozilla firefox, but also with 
safari, IE and even opera.
Generally, it turns out to be a dns lookup failure.