Proxy/network configuration prevents yum and firefox but not konqueror (FC17)

[Don Levey]

So I can't wrap my head around what I might have done to create this
situation, or to back out of it.

I have one machine on my home network that I protect via a squid proxy -
my kids' machine.  Somehow, something I (or they) did created a
situation where I cannot connect externally via Firefox, and cannot
connect to any repository via yum.  However, Konqueror doesn't seem to
respect or use the proxy configuration, and can connect anywhere without
restriction (regardless of the system settings).

I don't think it's truly the proxy itself, though.  I configured another
machine to use it without issue; Firefox and Yum both work without a
hiccup.  What's more, disabling or completely removing the proxy
configuration on the problem PC doesn't seem to alleviate the problem.

I'm not even sure where I should look to gather more diagnostic info;
any suggestions?

Thanks in advance,
 -Don