After FF has been running for a while (hours/days), I start getting the error "firefox is configured to use a proxy server that can't be found" I'm using a 'manual' proxy. When this happens, I can still resolve the hostname of the proxy (using nslookup) and can ping it by hostname with a response. Other browsers continue to use the same proxy successfully. Changing the proxy hostname to another proxy doesn't resolve the problem--I continue to get the same error. Changing to a proxypac file or changing to use system settings (same proxy.pac URL) also don't resolve the issue. I've not tried putting the proxy's IP instead of hostname in the config dialog.