Firefox caches proxy settings per hosts

It turns out Firefox now caches proxy settings per host.

This causes us problems when people reconfigure their proxy settings, and also during startup when we change our proxy settings based on the environment variables from Vidalia. This change in proxy settings sometimes happens after the browser has already attempted to connect to check.torproject.org for the HTTPS-Everywhere Tor test, which then causes check.torproject.org to forever use the old proxy settings.