My school district requires using their proxy server. Suddenly FF 16.0.2 no longer will connect to the proxy server, despite all settings configured correctly in System Preferences. Firefox prefs are set to "use system proxy settings". I've also tried Manual proxy settings, and Auto-detect proxy settings for this network. All of those options used to work just fine.
I CAN successfully connect using Safari 6, so it's not the proxy server itself (Safari only uses the System Preferences proxy/network setting).